A lot of the ebay ones are already used and washed.
you could always break into your local post office???
Staples the office supply people also do bulk discount of about 5% and so the post office if you buy enough
i always buy my stamps from superdrug they are great
I know that some suggestions were posted to this thread http://community.giffgaff.com/t5/Contribute-Innovation-Promotion/Stamps-microgaff/td-p/1832483