I collect the SHIT out of DVDs so let me give a few suggestions
depending on where you live
Big Lots is surprisingly awesome for finding stuff to complete a series for pretty cheap, stuff like Friday the 13th... franchises, things of that sort. I don't think I've ever paid over $5 for anything they've had. They usually have a pretty good selection this time of year. Some stuff I was surprised they had, "Thirst" and "Otis" never thought to find those there $3 each... Sadly I already owned them
Walmart.com... Seriously they have pretty sweet deals.
If you don't mind paying to import things normally OOP in the states go to
www.dtm.at think they are based out of Austria so you'll pay like $20 in shipping but I found stuff I've been looking for for YEARS there. You'll need a region free dvd player though.
Also try Diabolikdvd.com, sometimes over priced on things but it's very likely they'll have what you're looking for
Also, Pawn shops. I've found some great stuff in pawn shops and generally stop at any new ones I pass.
hope this helps.