Here is a directory of some DVD & Blu-Ray Disc Suppliers. I have used some of these to source products to resell on eBay, and others I have never used. Some have a great selection and others only have a few.
Ingram
Baker & Taylor
VPD
A&E Bulk Lots
Brandywine Liquidators
Time/Life
Graveyard Mall
Glory Days Entertainment
Frontier Video
Wax Works
Buy-Rite DVD
DV&A
First Video [...]

The site with the lower prices on DVDs & Blu-Ray discs on the internet is Deep Discount. This site charges NO SHIPPING and you can sometimes find amazing deals especially on some of their sales. Make sure you sign-up for their mailing list so you can be alerted when they have a sale. [...]

Do not discount the fact that you can find DVDs and Blu-Ray discs locally to resell on eBay. This has been one of my most successful sources of find good used DVDs.
One of the best places to find DVDs and Blu-Ray discs locally is at a pawn shop. Pawn shop’s are able to [...]