Small stuff like Jify envelopes etc try looking on ebay.
larger quantities, more specialist stuff, try Aid-Pack
Polythene Bags, Mailing bags, Carrier bags
Free delivery on all Plastic bags, polybags, mailing bags and carrrier bags. Cheap Prices & Free Delivery, buy at Polybags Limited, UK
My favourite, but check the offers carefully – sometimes you can get a bargain if you look, but they don’t tell you if you don’t!
Originally recommended by Tony, the prices here look excellent for both blank optical media and ink cartridges: