Whenever I asked TST for o-rings, they quoted a high price and I just paid it to get the project done.
This year I decided to look for o-rings elsewhere. I always wondered why TST provided tiny o-rings that need to be stretched SO MUCH to fit. When it's time to replace them a year later, they always crumbled off. That never seemed right.
Anyway I found a set at the local Home Depot's plumbing aisle that fits much better (way less stretching required) for a lower price. It's the Danco #30 O-Rings (10-Pack), model number #96744, store SKU #584133.
I've only had the Home Depot o-rings on for a few weeks, so I don't know how well they'll work in the long-run, but I like the way they fit.