My son's shop has got both cassettes and chains. He is not on LFGSS but you can email him directly at stig5shed@ntlworld.com. Tell him you are rebel from LFGSS and that his Dad gave you his details.
I've just been through about 20 cantis and, as you say, most springs are 17mm+ OD. However, I did find one which I measured at a smidjen under 16mm. I've just put it into a jar of acetic acid to clean all the crap off and will measure it again in the morning. Looks promising, but there is only one.
My son's shop has got both cassettes and chains. He is not on LFGSS but you can email him directly at stig5shed@ntlworld.com. Tell him you are rebel from LFGSS and that his Dad gave you his details.