Hi everyone, can you please tell me how and where I can buy stuff in TF2? I've heard that there are other third party sites besides the in-game store, but I don't know how to find them.

Yes, besides in-game store in the Team Fortress 2 game, there are indeed a huge amount of other side trading sites in which you can purchase items and various weapons and equipment for your player character in the TF2 game, and it's not hard to find such sites. For instance, here is one of them - https://tradeoffers.tf/orders/440.