And where can I find it online? I see this in a few posts recommending to add it to citrus plants. Is this a product on the market or a mix one can make on their own?

Soluble Trace Element Mixture-- sold by Peters in 25# bags I think. I have seen it on Ebay. I got a fairly similar mix called Essential Element Mixture at a local Ace hardware 5# for about $6. It does not take much even for large trees.

I purchase STEM (Soluble Trace Element Mix) in the 25-lb. bag. If you would like a pound (which is a lot of STEM) send me a private message and I will sell you a pound. STEM is used at the rate of 25-grams per 1 cubic foot of potting mix when making up new medium. Also used at the rate of 0.4 - 1.3 grams per quart of water as a foliar spray. Spray the foliage to run-off. A spreader sticker should be added when using as a foliar spray. - Millet