If round pots are acceptable, check these out:
WWP Resources 75 New 3.5" Nursery Plastic Starter pots
$7.50 + $4.49 shipping
Top diameter: 3.5 inches / 90 millimeters
Bottom diameter: 2.25 inches / 60 millimeters
Height: 3.125 inches / 82 millimeters
Number of drain holes: 6
They're sturdy, durable pots with the same volume as a 4-inch azalea pot. I showed them to my friend who owns a commercial greenhouse, and he was ecstatic with the find. Between us, we've bought over 2,000 of them. FYI - If you buy multiple sets, the shipping is still $4.49 for the entire order (at least, it was when I ordered my last batch 6 months ago).