http://www.mcmaster.com/nav/enter.asp?pagenum=3161
how about the 3/8" phenolic size on the above page? I know the dims are a bit off, but you could open the ID and thin them if the OD works. Also the page gives you a nice idea of what temperature ranges different materials have
Also take a look at the PTFE (teflon) and Nylatron washers on page 3160