Just keep an eye out. The banzai setups pop up every once and a while. Look on local rider forums, this and other RC51 forums, Craigslist, ebay, www.rc51parts.com. I've seen sets for sale in all of these places over the last year.
I wouldn't expect to see a set of mids by themselves though. Can't say I've seen that. Be prepared to buy the whole shebang, and ready to pay quite a bit more than any other used exhaust for the RC51.