I've heard quite a bit about skin.club. It's one of the more popular platforms that offer custom CS2 cases that aren't available directly through Valve. They have a massive selection, from cheap farm cases if you just want to casually open something, to premium knife and glove cases for those looking for something a bit rarer. Plus, they've got some interactive modes like upgraders and case battles, which add an extra layer of fun to the experience. They also have a level system and offer daily free cases, so active users can get some nice bonuses over time.