Go Rummaging
Anywhere that old toys may be found or sold is a good place to start. This would include yard sales, rummage sales and even a family basement or attic. Please get permission to look around first. Most marbles you find won't be worth much, but you never know where a treasure might be hidden.
Go "Antiquing"
Anywhere you would look to purchase antiques is another good place to search for valuable marbles. This would include flea markets, malls and antique stores.
Marble shows
There are many marble shows put on throughout the United States. They can have as few as five dealers or as many as 100. They will all have opportunities to purchase valuable marbles. Many of the shows are put on by marble clubs, which you can join, usually for a small fee, and they will send you flyers or newsletters with information on their show as well as others in the area.
Auctions
Just like any other antique item, marbles can be purchased at auctions. Some antique auctions will include some marbles, but there also are auctions that deal exclusively with marbles. There also are online auction sites such as eBay that will allow you to purchase just about anything.