If you're lucky, you can find one for sale :'D
You'd have the best luck searching on livejournal communities rather than ebay. There are some pokemon collectior communities that might have someone selling one.
But since it's pretty hard to find now, I guess... Well, I would modify a normal watch with clay. You could either use an airdrying clay, or for something more durable you could work with a polymer clay like sculpey or fimo. Then you would have to take it off the watch to bake it, and glue it on the normal watch in the end.
