I'm handy and willing to work a bit harder in order to save some cash. I built a bat house to catch it's droppings and I think I'm getting some bunnies but I want to do/learn more than just manure.

Spoiled milk makes a great foliar spray or soil drench. You can dilute it (3 ounces of milk per gallon) and spray it or pour it on. The milk feeds the microbes living on the outside of the plants.

Usually you can get free horse manure from a stable. One of the old regulars here, CaptainCompostAL, started a compost business based on all the free horse manure he could carry home from a local vet.

Also keeping trash cans of mineral and nutrient rich teas like comfrey, nettle, dandelion, yarrow leaves brewing in summer. You can grow a patch of these perennials easily for this purpose, free after the initial purchase of seeds or seedlings. Careful to keep them in bounds though.

Volcanite is one granite product.

And by all means, please mention brands, store names, and locations. That's a lot more helpful than beating around the bush.

A pound of alfalfa seed can be had for under $15 with shipping, often cheaper. This will plant a very large area, something like 1/4 acrea if properly spaced out. Not everyone, of course, has the space to do this sort of project, but if you do, its a good way to get a lot of high nitrogen organic matter for your gardens.

Hay, IOW. Mulch hay is a pretty cheap way to to just that, for those that don't have the time, energy, or space to cut their own by hand. There is typically a lot of grass-seed in hay but if you pile it on beds very thick for the winter the spring sprouted grass is not well established and quite easily turned under. I usually pull the partially broken down mulch off the beds and throw it in the compost or made windrows of it between the crop rows. One can probably get that 1/4 acre's worth of grass for under $20 if you get spoiled hay.

This sounds ridiculously time and labour intensive but has anybody tried it?

If you compost and add that to your soil and mulch that soil with those deciduous tree leaves that fall every year (unless you are someplace they do not grow) you will not need to spend money on "fertilizers", after a period of time if you get sufficient levels of that organic matter into your soil. After 35 plus years of adding compost and mulching with shredded leaves I have to be careful how much of each I add since my soil pH, which was 5.7 when we first moved here, now tends to be 7.2 and the P and K levels are in the high optimal range. All the animal manures have been composted before being added to the planting beds, never have I put uncomposted manure there.***

Are you saying that the pH went up as a result of all the organic matter over the years?

Composting and collecting opl (other people's leaves... use whatever euphemism you like) are superhero-like socially beneficial activities. You save landfill space, educate your neighbors, increase biodiversity, provide habitats, enhance topsoil, enhance greenery, and thereby pull CO2 out of the air, etc. Like superhero activities, they also take time from your personal life, which can be expensive in non-monetary terms.

Anyone could save a lot of time and trouble by ordering up a dump truck load of whatever. I did not do that. Somehow it would not have been mine, monetary sacrifice notwithstanding. Even taking bags of leaves from someone else's tip or curb involves a sacrifice and personal statement beyond opening a wallet or purse. There is certainly no need to buy anything for your garden.

True frugality is serendipity: Minimal funds AND Minimal effort. Use what you have and take the low hanging fruit. I think 20-30 gallons of human urine and 5 cubic meters of compost a year are within anyone's capabilities. That alone could sustain a decent garden. Using cover crops and green manures in off seasons would eliminate the need for fertilizers, I would bet. I am pretty sure that academic studies have shown that... even for moderate to heavy feeding crops.

I think nobody mentioned it above, but saving bean, pea, and mustard seed is easy as falling off a log. There are your green manures right there. For free. You can have those cakes and eat em too. That is frugality. I bought some small seeds intended for sprouting ... for salads... they were very cheap crucifers, so they kept me going until I built up seed stocks. Vetch and clover are so common where I live that I probably should not have bothered. What is cheaper than weeds?

I have a nice garden built up almost entirely from compost (opl, weeds, garden residue, kitchen waste, wood ash, human urine, wood chips, sawdust, shredded cardboard), with smaller amounts of vermicompost, chicken manure, pond scum, sod, and disused potting soil. I will likely switch from OPL to green manures because of the increased use of opl biomass for community purposes in my area and because I want to spend more time with my family... even though being a superhero was fun while it lasted.

For micronutrients; rock dust from granite countertop companies.

For sources of lignin; tree service companies.

For phosphorus; produce culls from food stores.

Clay soil conditioner; coffee grounds, in moderation.

All of the vegetables I can use this year will be produced for an out of pocket cost of less than 30 dollars, and that is because I panicked and bought some plants instead of waiting for my seedlings. I also need a new handle for an important tool.

Everything you need is available near you. Scrap lumber or stones or bricks or cardboard for raised beds. Compost from kitchen waste and leaves and weeds. Existing dirt, preferably topsoil, from somewhere.
Other free stuff you could add could be anything. Shredded paper. Cardboard. Sticks and twigs. PET bottles. Human urine. Wood ash. Chalk dust. Dead animals. Cotton rags.
Get seeds from other people's flowers. Always ask. They will laugh at you. Get seeds from foods you eat. Take cuttings from plants.

It is a handicap that makes gardening extremely interesting for me. Frankly, money is not the problem, but I have relatives who have spent more to grow a few tulips than I have spent on my whole garden.

I use old planking with paint peeling off for raised beds. If I used new lumber, I would think of dead trees. I make my own compost... all of it. I buy poultry manure in big bags when it is on sale. Maybe five bags at a time. And I used to drive around in a truck at four in the morning snagging people's bagged up leaves and weeds. (I counted all of the bags of leaves and weeds I have saved from disposal in landfills. How many? More than 600.) I reuse old plastic pots and planters until they fall apart. I use tin cans.

Start today with what you have.