As with the Jazz Record Center, you just have to keep your eyes open in every possible shop that may carry them. The bulk of my Classics were purchased for $5. And I kick myself when I think of the dozens I didn't buy because I just didn't know how good they are.
Also, buy every one you see, whether you like the artist or not. They can be traded or sold for other titles. I just picked up 9 last week.
The great elusive ones? The Benny Carter 1948-1952 (1297). I know it will be the weakest of the Carter series, but I still want it. After that, the Lil Hardin Armstrong. That shows up occasionally, and next time it will be mine.