One good source for used LP’s are vintage malls, where there are multiple vendor booths under one roof. People from all over the U.S.A. have made YouTube videos showing the mall at which they have found LP’s, with wildly-varying prices. I have one ten minutes from my house that has a permanent vendor who specializes in music, and his VG+ and M- LP’s are priced for the most part between $5 and $10.
If your taste runs to 1970’s Rock, the Warner Brothers group of labels (WB, Reprise, Asylum) are some of the best for both music and sound.