giffgaff vouchers can be bought in most shops providing epay, payzone or paypoint services. Don't know where to find one of these shops near you? Check thisarticle for more details. You can also find them in Post Offices and most supermarkets.
If you can't find a giffgaff voucher at your favourite shop, you can get an O2 voucher, they also work. If you buy a voucher for O2, make sure to follow the steps described in the next sections, and not those printed on the voucher itself.