HG,
Sounds like you are talking about the brake accumulator and not the reservoir.
About a year ago, somewhere on Turbobuick.com the p/n of about 5 currently available accumulators that will work with TR's and cost $50 or so were listed. However, I can't attest to those claims as I only had to change mine once and that was about 15 years ago. I used OEM.
Good luck.
Rich