Gear can be purchased from any number of locations online, or in person. If you're looking for something in particular, it might be easier to tell us what you want and we can suggest retailers for you. Specifically for trousers/tops/etc., so long as you're getting the same brands, I would suggest searching the web for the cheapest option. Otherwise the cheapest option will most likely be the used market, either on this forum, other forums, or surplus stores. In reality though, there are so many, many options that you will need to take some time searching the web to kind of learn the ropes a bit. I've acquired my links and knowledge after a lot of years, and time, of research and tooling around the web.
Medic rules very by field and even by game/host, so you should ask questions regarding those (and other) rules in the specific threads for the game you wish to attend.