Do a web search using the words "liquid chelated iron" and lots of options should come up.

Are you sure you need it? Most fertilizers you might use for houseplants contain more than adequate amounts of Fe (iron). Additionally, most houseplant soils, whether peat or bark based are unlikely to be actually deficient in Fe. I use the word 'actually' because there are instances in which Fe can be present in soils, but 'unavailable' for uptake. As soil (solution) pH rises, Fe becomes insoluble and thus unavailable to plants. Aging soil, which is usually accompanied by rising pH, and instances where your water is high in pH or alkalinity can cause this type of deficiency. Generally, all that's needed to correct the deficiency is to add an acid to lower pH. A tablespoon or two of vinegar in each gallon of irrigation water normally does the job nicely for me. It lowers pH, making the Fe that was there all along available (soluble) for uptake.

Though probably not applicable, antagonistic deficiencies of one element can be caused by too much of another element in the soil blocking uptake. High levels of Mn (manganese) in soils can block the uptake of Fe. Correspondingly, too much Fe in the soil can cause a deficiency of Mn, so you should always be careful about adding Fe w/o adding Mn at the same time in a favorable ratio.

When I have and identify an Fe deficiency, I can usually be pretty sure it's being caused by my high pH water. When it's not practical for me to add acidity in volume (summer - watering from the hose) I use "Sprint 138" in my fertilizer solution. It's a chelate "key-late" that is especially formulated to perform under high pH conditions.

Al

Oops .... the Sprint is 'specially' formulated to perform ....., not 'especially' formulated ..... ;o)

Al

JE, it would be better served if you were to identify the plant whose leaves are yellowing.
Some plants, notably those that enjoy an acidic side to their soil, often meet up with a deficiency of iron which, if treated properly, can cure the problem.
However, when the plant is not deficient in iron, the addition of the chealted iron will prove harmful.

Its much better to think other than a chemical reason why a plant drops a leaf or two. There are much better reasons to consider than fooling around with nature.
For instance, a bottom leaf turns yellow on a houseplant, the most common reason why it does so is --you...
you might be overwatering, underwatering, not giving the plant sufficient light, or placing the plant in a location where airstreams from an open door, a window or a heat vent or, if the plant is growing well, not sufficient fertilizer.

Sometimes too, too high a nighttime temperature can cause such yellowing. Plants usually enjoy temperatures in the range of 60 - 65.

All the above may apply and you should dispose of each before going the extreme in thinking its in the soil.
Which brings up another possiblility....bugs.
Houseplants are commonly attacked by many bugs that causes leaves to lose their photosyntheses--yellowing---and they drop.

Its not practical to get a soil test done on such small amount of soil, but before you again think its a chemical problem, do the simple math first.

I have 3 local nurseries that carry these brands of organics:

http://www.growmore.com/productpages/natural.html
growmore does have a powder organic chealted iron

http://www.scotts.com/smg/brand/whitneyFarms/whitneyFarms.jsp?detailId=600012

http://www.ebstone.org/ebstone.php