Ok serious answer.
Deepdiscount is great when they have their semi annual sales, though they mark up the prices on some DVDs before the sale starts, so you would need to check and make note of the prices in advance. Normal prices are OK, but other places might have it cheaper.
Amazon is great if you have prime. Some of their sales are nice, but not frequent.
TSRI is awesome with clearances. ADV also has massive clearances from time to time, but they remove the barcode from everything.
Of note, I no longer buy stuff, as I like my shit in HD.