Schreiners is certainly "iris central", if you want catalog ordering. Be aware however that with irises it's even worse than with most flowers, in terms of the pics being um not extremely realistic and with the colors often rather firmly tweaked
(Schreiners' catalog isn't as bad as a lot of them on that, but it's still an issue)
If you have a local (or "reasonably local") garden club or horticultural society, their plant sales or swaps are often a good way to get irises for much cheaper (per size of plant).
You wouldn't want to be acquiring/planting irises til later this summer ANYhow, though -- usually one divides or moves them a month or six weeks after they finish blooming, or up thru early August outside Philadelphia anyhow.
The main things to know about bearded irises (which I'm assuming is what you mean, that's "the iris" most people are talking about):
1) they hate wetness and need good drainage, tho are only "somewhat" drought tolerant. In heavy or questionably-too-damp soils plant with almost all the rhizome atop the soil; in normal good garden soil plant with the top of the rhizome sticking up a tad above soil level. If you plant them somewhere too wet, they will simply rot away. (e.t.a. -- a site that's a bit too damp for bearded iris will be just right for siberian iris, tho! And for a really boggy site there are really boggy other species of irises)
2) if you are the only one around who has bearded irises, and you don't have too many, you may well have little or no trouble with borers (at least for a while). If however you get seriously into irises with lots and lots of them around, in Pennsylvania expect to be locked in permanent battle with the borers. With vigilance and removal of affected parts, you can hold your own, but they are not the very most problem-free plant in the world when grown in number.
3) most varieties require division every few years (the ones that don't "require" it still APPRECIATE it, in terms of giving better flowering). The way irises grow is the rhizomes grow horizontally across the soil surface, in segments, each segment giving rise to (I forget exactly) one or two or three fans of leaves, each of which will bear a stem or two of flowers. When the flowers are gone the segment then gives rise to another one or two, growing outwards from where the iris was planted. The older segments do not bloom and may not even have leaves. Thus if you neglect a clump of irises you tend (with most varieties, IME) to end up with a big circle of foliage with a bare (rhizomes-only) middle and flowers only around the edge. The rhizomes will also grow up and over each other to some extend when very neglected, which doesn't produce good flowering and is a mess to try to disentangle.
When you buy a mail-order iris and it's just like two fans or so, it will "probably but not necessarily" bloom the next year despite not taking up much garden room yet, but in a couple years you'll be dividing it and making more plants from it anyhow. They do grow reasonably quickly.
4) tall varieties don't do well in windy spots, especially if it's also part shade (making them taller and spindlier). It's real hard to stake irises inconspicuously. You can still plant them there of course but you gotta expect them to flop over. If picked *immediately* and put in a vase the flopped stems are fine cut flowers; if you've left them there for 12 hrs or more though they tend to take a permanent set and you have horizontally-flowering irises. I don't personally mind this (it's the way a lot of mom's irises always were when I was growing up, LOL) but some very ordered or artistic gardeners find it upsetting
If you want a "plant it and forget it" perennial, I'd suggest something more along the lines of peonies, or Siberian irises if you like irises. That said, bearded irises are not AS high-maintenance as some things -- borer patrol is the main thing, once you start having borer problems. Division can be let go a year or two and it's not like you'll LOSE the plant, it just won't do as well -- and they are extremely, extremely beautiful. Some are also nicely scented. I particularly recommend Immortality which is a scented white rebloomer, what more can you ask
Good luck, have fun,
Pat
