The Guam US quarter has been made as a special silver proof by the San Francisco mint, much sought after by collectors and investors alike. It is also possible to find multi-coin sets and large lots of silver Guam proof quarters in bulk if you watch the current listings closely.
Check the current offers to see what is available now. Note that there are also a few gold-plated Guam coins available sometimes, though these are not pure silver like the official Guam SF mint proofs. Check back often; selection is updated daily.