I am not sure if you can really buy them anymore and from where! But there are some tutorials that shows you how to make a homemade board for this, or maybe do this without a board as well(but it will be a little messy maybe?)

Here are some different tutorials from acidmods(not sure if I am breaking a rule or something... if yes sorry, just tell me and I will delete them...):
http://www.acidmods.com/forum/index....c,34880.0.html
http://www.acidmods.com/forum/index....c,27423.0.html
http://www.acidmods.com/forum/index....html#msg282228

The whole idea is this:
The psp's stick has a small spring on it, which you can use it as ground and use the other four pins/metals for making it work as 4 keys

By the way kimmoli I like the way you are thinking