Hey all! For you guys who like using the NES Advantage, but want something just a little different, I just took mine apart and the stick looked like it would accept a Sanwa/Seimitsu balltop, so I gave it a shot and it works! My Advantages are in pieces at the moment as I'm cleaning them up, but I can post pics in a couple of days if people want to see what it looks like. The arcade style tops are larger than the original Advantage top. You can also put a "bat-top" on if you get an adapter (they are really cheap).
There are many sites to find these, but here's one in Southern California if you want to browse through the options.
http://www.arcadeshock.com/items/sanwa- ... -/list.htm
Edit: Here are a couple of photos on a disassembled stick for comparison.
Left side is the original size.