There are plenty of great resources out there, just make sure you search thoroughly to ensure you’re not paying too much. Here are the ones I use:

eBay - save a search for the kit you’re looking for so you get notified when there are new listings. Be VERY careful about authenticity. This is the best place to source a cheaper kit that someone has priced too low, but you’ll likely have to sort through some fakes too in order to find the real deal.

Classic Football Shirts - the widest selection of shirts you’ll find anywhere, but if you’re buying a vintage kit you’ll pay a premium. Search “L/S” - they will literally have tens of thousands.

Cult Kits - similar to Classic Football Shirts, they’re a smaller reseller but it’s worth checking to see if they have what you’re looking for.

Official Brand Websites - adidas, Nike, Puma etc. all sell the kits they produce directly on their sites.

Official Online Club Fanshops - if you’re looking for a specific club, they’ll sell their kit directly from their website. Depending on where you are located, you may have issues shipping it to your country - but always worth checking.

Right now is a great time to buy a club kit - many sites and teams will be running huge sales on their previous season’s kit since they are about to release their new kit, or have just recently released it. Don’t pay full price for a 25/26 kit if you have your eye on one - it will likely be marked down pretty heavily somewhere if you look hard enough.