I've got my own durametric kit so I'll test anything I look at. I suppose this is the best insurance I can do. I run a garage so I have a reasonable idea of what to look for, I'd just hate to miss something major.
As for where I'm finding them, I'll let you know after I buy one.

Really though, I'm just looking all over the place, PCA websites (Toronto and Ottawa regions) and trader.ca. Just have to look regularly and sort through all the fodder.