The PRI is much harder to deal with, find, and flash compared to a PRL. The ONLY available Sprint PRI is the one that's included with the "Updated Sprint" stock rom, in the stock ROM thread.
http://forum.ppcgeeks.com/showthread.php?t=42741
There isn't really a PRI download list that I'm aware of. The stock Updated Sprint ROM should put you at 1.53. I don't believe there's any way of going back to 1.47. PRIs are a tricky thing, and switching them can cause big headaches if something goes wrong. PRLs shouldn't cause any overheating issues unless you're on the edge of coverage area. The PRI however might.
PRL - Preferred Roaming List (Just gives the radio a list of preferred cell towers)
PRI - Preferred Radio Instructions (Tells the radio how to operate)
If you get the wrong PRI, you can see how it would seriously cause problems... far worse problems than overheating.
