Some of the online sites might be OK but there are two problems with using them. Firstly you need to take the stereo out (which can be a very fiddly job). Secondly if the code you're given doesn't work you might have lost your money. (You'd have the legal right to a refund but you might have a big fight on your hands to actually get it).
Until recently I was working for a Ford main dealer. As long as the stereo was the original one fitted into the car, all they needed was the vehicle registration number. Then they could get the code directly from Ford. The usual price charged (including VAT) was £15 but (for reasons which were unclear to me) some codes only cost a fiver.
Chris