Packaging
Small stuff like Jify envelopes etc try looking on ebay.
larger quantities, more specialist stuff, try Aid-Pack
Boxes:
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
Stationery
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!
Office Consumables
Originally recommended by Tony, the prices here look excellent for both blank optical media and ink cartridges: