Ive not done this myself, but if I was going todo so it'd be this way:
Personally, Id go with sticky sided paper you can print on a normal A4 sheet, and with that also put a Clear Matt Transparent Self Adhesive Vinyl Laminate sheet over the top, cut them out an slap em' on the side of your PSU. the plastic laminate makes them look a bit more high quality too.
The only other alternative id think is to order it online, but its a bit risky considering you can see what its going to turn out like until you wait however many days/weeks for it to arrive.
Hope it helps!