I generally use the "heft test" when buying potting mix......and potting "mix' IS different than potting "soil"......When you are shopping in your "nursery", or home store, grab onto a bag of "topsoil" and pick it up. It's pretty heavy ! That's because it contains a lot of "mineral soil"....sand, silt, etc. Then, find a bag of shredded sphagnum moss, and lift it....it's very light, because it's all a low density ,all -organic product. What you want, is some thing about half way between those, in weight. The 'all organic" WILL work...it's very loose, holds moisture well (and still drains excess water fairly well), allows for easy "root growth", and ,of course, doesn't pack down, and become a "brick" in the pot. And the " topsoil" will work, as well.....that's what they are going to grow in, when you finally "plant them out", after all......but in "pot culture", it's going to be too 'stiff ' for water penetration, and easy root growth, and require much more attention as to watering........when and how much.....to keep the iris happy. As to any "added' fertilizers.......some potting mixes have it, and some don't....I don't think they are terribly important.....you can add a little of your own fertilizer (sparingly !) or water with a dilute Miracle Grow solution, every 3rd, or 4th time you water. Perlite is added to lots of potting mixes in order to keep it lighter in texture, and reduce 'packing down'. But , if the mix has enough sphagnum moss, or shredded bark....they do the same thing
I usually buy my"mix at a local Manard's ....it's their own "brand', and a big sack is about $11 (I think ? I really don't remember...so it must not be TOO expensive
), and when I get "out and about", I'll read the "ingedients", and post them. I often reuse it, after finally planting the iris, to pot up the next batch....and, sometimes I "plant" the entire contents of the pot, when I plant the iris....if the dirt I'm planting in needs more 'organic content, or just leans to the "stiff" side.....it can stand the amendment of some" loose" stuff.
Bottom line, IMHO, is it's not rocket science....and, in most cases...it's only a temporary thing, as you will be planting them out, after a month, or two, so , really almost any commercial potting mix will do fine (as will any "home made" mix that approximates a commercial blend), so, depending on how many you have to deal with, $10-$15 worth of mix will go a long way, and , as I mentioned, can either be reused, or incorporated into your beds as an amendment.