prabbit, you first have to distinguish between Boost w/ PTT and Boost Unlimited. The former will work anywhere that has an iDEN (aka Nextel) signal, while the latter should work anywhere that has a CDMA signal.
If you're looking at Boost Unlimited (which seems to be the case, since you brought up MetroPCS), then Boost's coverage is the same and Sprint's CDMA coverage. So you can very readily go to any local Radio Shack, WalMart, or SprintNextel store (check your local mall, they probably have one) to get a Boost Unlimited phone package.
Also, Best Buy also offers Boost as well (though I'm not sure if they can do the Boost Unlimited...haven't bothered to ask).