Lowe's, rarely Home Depot. For seeding trays and the like, usually online - Park's or Gardener's Supply or Burpee.

I used to get nearly everything from Mellinger's but they've been out of business for literally decades now. I still miss them. *sigh*

If you're in an area that has a real nursery (eg one where they ACTUALLY GROW THEIR OWN PLANTS), check there.

I know the pots that these plants come in are not the best but I do make do with them. When buying clearance plants from Lowes and other locations, I often think --- I get a plant, and a pot!

And that turns out to be the case as the plant may still die, or survives but dies in the coming winter -- so then I have a pot to re-use.

The other way I have gotten tons of pots are from friends IRL and strangers on craigslist. Those persons do make their own landscape projects every year, and while they could return them to places like Lowes for recycling or, as is often the case, people just throw them away, but if you make a request for pots, you may find that you'll get plenty to last for a season or two.

I've switched entirely to Root Pouches. Greenhousemegastore.com has the best prices. I have hundreds of them now.

I've found an online source and just bought some pots and plant trays from them. I felt they were very reasonably priced. The shipping wasn't horrendous either. Like $8 for the 2 trays (31x16), 20 4" pots, and 10 3 gallon pots - good ones, not the flimsy black nursery pots. Same place as mentioned above.

Here is a link that might be useful: Greenhouse Megastore

Oh I am wondering how long you have been using the root pouches? I want to buy them too, but I want to be able to use them for more than one season. I had some hi yield pots I purchased from pine tree seeds last year, however they fell apart. They are in the compost now, but I would like to get at least two, hopefully three seasons from these pots. Please advise if you used them for at least a season.
Did you get the brown or grey? The brown is supposed to be the longest life pots I believe.

I used root pouches one season and they look new. Leaving them outside for the winter, and I get winters here. I have obtained about 50 pots from going around in the spring and fall on garbage day. Some are fanatstic. I needed a very wide pot, about 2 feet wide and 1 foot deep. I found 3 of them in the trash! 2 slightly smaller ones were in the trash too. I need them for cacti. I have a giant brain cactus and grow it each year in a bird fountain outside. No pot would fit it. I would just put it on plastic for the winter. Well now I have a pot. Too bad it broke up on me before I could get it in! Oh well, now at least it can stay in a pot! Yeah! I got three of these pots now and two others that are about 18 inches wide, about 1 foot deep. I have been planting annuals in them, but will find a better use. In perfect shape too. Love it!

I look on Freecycle , the flea mart, yard sales, craigs list. If you want new, I would go to the Big Box stores or somewhere like Biglots. Nancy

I have 5 cats, so I save all my kitty litter buckets and turn those into planting containers. Kitty litter jugs make great mini greenhouses for winter sowing. When holiday party tubs go on mega clearance, I try to get some of those for bigger pots. Or when those holiday storage bins go on clearance, those make great containers for bigger plantings.
I keep my eye out at the dollar stores and thrift shops for cheap pots too.
For smaller plantings, I recycle large butter tubs, 5 pound sour cream tubs from GFS, and gallon ice cream buckets.