itch.io

Yeah, buying them in bulk lessens the overall cost, but it's still incredibly expensive. Alternatively, you could consider buying a USB cart, such as an Everdrive or the Ez Flash Jr. You can put multiple roms on them, and they are of course compatible with custom roms too. It's not as fancy as having a dedicated cart for a game, but still an option for playing these roms on the original hardware! :)