Where can I find replacement pads for a bicycle helmet?
Visit the friendly folks at your local bike shop at a quieter time of the week and ask. Chances are that they will have plenty of spare pads tucked behind the counter. Buy something else at the same time and they might let you have them for a favourable price.

This is one area where a local bike shop can provide the service that you do not get shopping online, where it is all too easy to get the wrong bits and pay postage for the privilege.