Yes, you should be able to get the iron pills at the drug store. Plants can still get iron chlorosis in bought potting medium if the plant is particularly demanding, it may be that that plant has a problem getting enough for some reason (a "defect" as it were). If the pattern is reversed and the older leaves go paler on the other one we may be barking up the wrong tree but it's an easy thing to try. If you killed other plants with iron previously is it possible you applied too much? It's easy to do that with micronutrients. If you just pour some of the iron in water onto the leaves that should get the quickest result and if the leaves are damaged by it it won't kill the plant (although it doesn't look like it has a whole lot to lose at this point).
