I think you get the best deals at Iris Society sales. People in your area grow irises well-suited to the weather, and they are cheap.
The biggest, fattest rhizomes seem to come from Schreiner's, but they are not the cheapest. If you are buying a lot of irises or wait until their summer sale you can get good deals. I have bought from Stanton's, Wild Iris Rows, Cooley's, Blue J, and Schreiner's, and I have been happy with all of them. Sometimes there are minimum orders or shipping and handling costs, and some of the outfits are family-run and quite small, and they stop taking orders early and you have to wait for the next season. I do not buy irises from the big box stores or from Ebay. Too many horror stories.
Renee

A new one on the market is Burgard Iris Farm. Great prices so far and great plants.

BlueJ is closed for this year. They have a huge selection and excellent prices so bookmark them for next year. Nearly all my rhizomes bloomed the 1st year! And all were true as labeled. If you don't want extras deduct 10% from your order! My kind of place.

Here is a link that might be useful: http://www.bluejiris.com/

Thanks!