AFAIK they don't allow non-Sprint branded phones on their network, AFAIK. Nor are their handsets 'unlocked' in the same sense as GSM phones are. The carrier (in their case) controls who/what they let on their network.
To be honest though, actually answering your question it tough in any case since you fail to say what sort of 'cellphone' <ahem> exactly meets your specifications(?)
My contract is up on the Sprint network. One of the good and bad things about Sprint is the lack of selection. Right now, there're really only two options in my eyes, however, I was thinking maybe something else. Any ideas on a great cellphone that'll work on the Sprint network that's not exactly "sold in stores" or maybe unlocked? Something, anything.