I think June has the right idea. If you can find iris in pots, and not just the rhizome or bulb, you should do fine. Most iris with rhizomes (like the bearded types) have roots that initiate and grow vigorously at certain time(s) of the year, and not so much at other times. Spring, and especially fall are not a time for expansive root growth. Roots do grow, but starting with virtually nothing from a dried rhizome means they don't grow much at all. That's why potted plants are the way to go at these times. However, if you dig iris in the spring keeping roots intact, they will transplant easily.
Some rock garden iris that I grow:
Unknown I. pumila hybrid - 4-6 inches
Iris pumila - wild form -12 inches
Iris 'Spring Peeper' standard dwarf bearded - 10-12 inches
Iris attica (now Iris pumila subsp. attica) - this one is 6 inches, and the same plant the following year in a wetter season.
Iris suaveolens, - 4 inches at flowering. (yellow form and I. suaveolens var. rubromarginata)
summer/winter foliage (evergreen)
Iris tectorum - 12 inches
Iris chrysographes - 14-16 inches
Iris hookeri nana - 6-8 inches
Iris lacustris - 4 inches
Iris humilis - 6 inches
Iris lutescens 'Campbellii' - 10-12 inches
Iris hybrid - twice blooming: early spring 8-10 inches) and late spring (20-24 inches)
