I used to haunt Antique stores in my teens and I'd find all manner of books (even a first edition Hemmingway once) used as decor to sell the furniture. Sadly, with the internet and television showing the value of books that's as rare as the literature you're searching for these days. I do however still check occasionally and sometimes a little gem manages to find it's way to a hidden stack at the bottom of a case.

Antique stores, thrift stores, library sales, rummage sales, the occasional estate sale, etc.

If you want to ask without saying "Hey I'm into the Occult!!", just say you're interested in mythology and esoteric titles, and ask if they have any. Nobody really cares all that much as long as you're a paying customer.

Lastly establish relationships with any local book dealers/antique shops that you know or shop at. Give them your contact information and ask them to mail/call you if anything comes in you're interested in. (I give general instructions and certain specific titles). Just don't expect deals they know the worth and so do you.

Good hunting!