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).