Unfortunately, when these promotional cans are gone, it's almost impossible to find them. Personally, I have never seen them, and now I know why. The Monster cans were out in October 2000 for a contest to win a trip to Universal Studios. They were only sold in Michigan and were very hard to find.
Checking out flea markets and watching online auctions will probably be your best bet. In time, you may find them, but they will probably cost a lot more than what you would have paid for them originally. Save the ones you have and keep them in good shape. They will be very valuable later, if they aren't already. You're a lucky dog!
Thanks goes out to Danny Cheng's Coke Can Gallery for these rare pictures.