on ebay you can get hold of a4 sticky sheets, there usualy classed as labels-one per sheet, just type in adhesive paper etc, youl have to search. i usually get about 100 for a tenner , there real good, last for aggges.
If u have any contacts in the usa u can get hold of usps labels if ya ask nicely
as for the uk, postoffice have stopped the sticky orange signed fors.
best bet is to chek behind large supermarkets, where all the waste is, i found 2 huge rolls of waste control labes, about 6 inches by 4 and at least 500 on each roll.