There are many places to find garb. A lot of us use thrift stores to find basic pieces for our kits. If you keep an eye out, it’s not too hard to find cheap and useful clothing.
For those of us who are crafty, there are many patterns available online and in stores for making various kinds of garb. This is a cheaper route to go, but it requires an ability to sew, and the time to invest.
For those of use that do not have the time, money, or ability, there are many stores online you can buy them from. The resources page has many links to online vendors that sell costuming. If you want to support your local community, there are some of us in the game available for commissions!