The diaphragms on boosters will degrade if they are exposed to brake fluid from a leaking MC, so buying & installing a used one could be fraught with peril. Why not just buy a new booster? Admittedly, $400 is a big investment, so you could also get a quote on rebuilding your existing one (it'll bet it would be < $400). Karps could probably do it: http://www.karpspb.com
It's a fair amount of work to remove/replace a booster, and it would be a shame to find that your used booster doesn't work any better than your old one did. Or to have it fail after a few months.