I can't seem to find any plans that don't force me to buy a phone first. I already have a phone that I'm happy with, I just want to switch from Virgin Mobile to Sprint.
These two providers have different...uh... providers.
Virgin Mobile is using Bell's CDMA network and Sprint is using Fido's GSM network.
The technologies that the two companies use to send the data/voice signal to the phone are incompatible and therefore you would have to get a new phone.
Bell/Telus - CDMA
Rogers/Fido - GSM