My job involves a lot of driving around. There are a dozen or so thrift stores that I pass on a monthly basis or oftener. When I have a couple of minutes to spare, I stop in and have a quick look.
In a couple of years, involving probably well over a hundred searches, I have found 2 working Model Ms and 2 dead ones (not including a rubber dome 72H model).
I have found perhaps a dozen good mechanicals in that time, at least that many broken ones, plus some other niceties.
Assume that 95% of the time you go in, you will find absolutely nothing. If you are not driving around anyway, making a special trip will almost always disappoint.
Maybe try Mondays, on the chance that something came in over the weekend.
PS - I don't know about LA, but in the Atlanta area "Goodwill" brand stores don't carry computer gear anymore, because of pressure from real computer stores, according to the manager I spoke to.