Generally wherever you buy the triclovers should stock plenty of spare gaskets.
I grabbed my RIMS triclover and gasket from:<H4 id=YN3721x247710924_title>Barnett Enterprises Pty Ltd </H4>2 Croydon Rd, Keswick, SA 5035 (08) 8293 8266
No affiliation, but a quick search of 'stainless' and your...