unlocked GSM phones for sprint?

hi guys, I don't know anything about SIM cards, unlocked phones, etc. so work with me while I try to understand.

I currently have a sprint CDMA moto KRZR. I want to buy a new phone ( I will be paying full retail as I am not eligible for rebates)

Can I buy a GSM phone? I know sprint sells a couple. Obviously I could buy those, but if I choose to buy one that sprint doesn't sell, and it's unlocked will I be able to use it? Also since it would be a GSM phone, and my current phone does not have a SIM card, how do I go about getting one for my account?

Tmobile and ATT have so many nice phones yet sprint is laggin waaay behind on their phone choices. I would love to buy a phone that sprint does not carry (such as the blackjack II) and have it work, but I don't know if this is possible

Thanks for your help