place which sells iron for Rhododendrons
I need to find a place which sells iron for Rhododendrons and doesn't have extra chemicals. I had bought such a product at Home Depot several years ago, but they don't seem to carry it anymore.
First of all, iron is iron, doesn't matter what plant(s) you want to use it on. Second, in the matter of "extra chemicals," I'm guessing that you mean iron by itself, rather than as part of an all-purpose fertilizer. Probably what you purchased from HD was a chelated iron product, this is usually the best type of iron to use as a supplement. Of course, in these products the iron is also mixed with some other chemicals to render it absorbable by the plants. About the only way to get iron not mixed with other chemicals is to bury a bunch of iron nails in the soil around your plants; this is something people have done for a long, long time. If you want to get chelated iron, you can google "where to buy chelated iron fertilizer" for sources near you, or on the web. This article will tell you more about chelated iron: http://edis.ifas.ufl.edu/hs1208
Also, here are some more thoughts on affecting the iron in the soil: https://www.gardeningknowhow.com/plant-problems/environmental/leaf-chlorosis-and-iron.htm