Any sprint phone should work. Sprint owns them. You could get a decent one on ebay, and I think JMad has 75643 of them.
Sprint's phones are lackluster too. Low end androids or the Evo, which is on the large size, and the Epic 4G, which is nice, but Sprint requires an extra $10 a month for 4G. I wonder if it would work on Virgin.
I could probably ebay my iPhone for a decent amount to cover the cost of the new phone, and I'd love to go from $80+ a month to $25.