Thanks a lot for the feedback!
As for making custom cartridges, there are sites that allow you to order a cartridge with a custom ROM on them. The one I went to for my first custom cart was named Catskull Electronics.
You can also buy a custom cart flasher from Inside Gadgets, which is what I ended up doing, along with flashable carts of various specs. Keep in mind this will ship from Australia though, so there will be a bit of an extra cost for delivery (especially if you live in Europe).
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! :)
