I have been very pleased with the following (many duplicates of what others have said)
You might want to check a vender out on the Garden Watch Dog for comments about them.
In no particular order:
Iris 4 U - Nice sized rhizomes, often doubles, generous bonuses often new releases Excellent customer relations. Timely shipments
NAPA - Large rhizomes, nice bonuses, timely shipments, super web pages
Snowpeak - Nice rhizomes, nice bonuses, timely shipments, Super nice customer relations
Fair prices.
Mid-America - Nice Rhizomes, nice bonuses, timely shipments, large selection of new releases.
Keith Keppel Irises - Nice Rhizomes, Nice bonuses,(sometimes as many or more then the number you ordered!) timely shipments, great customer relations,
Bay View Gardens - Nice Rhizomes, Nice bonuses, Fills orders in the order he receives them, lower prices on older models.
Superstition Gardens - Nice Rhizomes (Extra large), Nice bonuses, Timely shipments, good customer relations,
Sisters (only ordered once last year) - Nice Rhizomes, Nice bonuses, Timely shipment, Good customer relations.
Schreiners - Nice Rhizomes, Nice bonuses, Timely shipments, good customer relations,
Suttons - Nice Rhizomes, Nice bonuses, Timely shipments, and good customer relations.
I've only had poor quality rhizomes from one vendor, and they have gone out of business. There are many more out there that I haven't used yet. I think there is a vendor list sticky at the top of the iris page.