The one I used is (was?) made by Mark Spaeth:
http://cambridgearcade.dyndns.org/?q=node/68
This must be an older design, as it looks different (better) nowadays.
You might find Mark hard to deal with. Many do. In fact, I am not sure he is selling anything at all right now.
Vector Labs sells it here:
http://www.vector-labs.com/index_dk11.html
If you don't have the rainbow cable, you can still use this, you just have to keep the power plug for the Video board plugged into the board. To switch games, you'd move the adapter + the Video plug to the other game.
If you want to have multiple boards that can be swapped just by pressing buttons here's the "base" unit for that, a 4-in-1 adapter:
http://vector-labs.com/index_dk.html
Plus you'll need one of these for each board:
http://vector-labs.com/index_dk1.html
At $85 for the 4-in-1, and $55 for each boardset, it can get a little pricey, but it is what it is.
I am using the edge adapter. Reaching through the coin door to pull the adapter off of one game and stick it onto another works fine. Being able to rapidly switch between games is rarely something I feel like I need to do. The much bigger problem for me is rotating my monitor sideways when I want to play Popeye, and there's no adapter or shortcut for making that easier.
