I was looking at these option initially too, but ended up going for the challenge since, both I read some not-so-good reviews and shipping the thing to germany would probably ended up being at least as expensive as buying the soldering equipment :) Building one of those custom GamePaks takes about ~4 hours once you have all the materials prepared (you really need very, very thin wires!) and putting it together for the first time is one of those finger crossing moments which you'll remember for a really long time :)

I took a cart, removed the game IC, and soldered in a ZIF socket for eproms. And I'm a klutz so it's easy enough.

http://www.devrs.com/gb/files/faq.html

Am I right in thinking that GameBoy carts are now a precious resource? We'll run out one day?

Pretty sure you could make your own with a 3d printer and one of those kitchen PCB-etching setups you see people using. There's no magic in there AFAIK.

The Everdrive GB takes micro SD cards, so it has more space:

http://krikzz.com/index.php?route=product/product&product_id...