I'd say bags and paper are generally cheaper off Ebay. If you can find a local paper/bag supplier, it might be worth checking to see if you can get small quantities.
If you buy off Ebay, try searching on location, nearest first and see if you can collect your paper/bags. That way, you'll eliminate the postage costs.
Jiffy bags are also available from Poundland, 6 small ones for a pound; four larger ones for a pound, so quite cheap.
I would generally wrap with brown paper, but I've got a humungous roll, so it's cheap as chips. If you can't get cheap brown paper, you don't have to wrap the boxes, as long as there's no address/confusing text on them. Even then, you could tape over them with white paper if necessary.
Hope this helps....other people might have better ideas. If so, this is the place to put em..