The least costly way to acquire that .500 S&W is to buy a new one. Not likely to find one used, and you're not likely to buy into an SB2 at a price that makes it feasible to spend that 'nother $150 to put a new barrel on one.
BTW, the SB2 has held up just fine with my 500 loads, including some "bruiser" jacketed 500 gr. bullet loads..