I love coming across small specialty stores like this. Cuddly Cavies specializes in costumes and clothing for guinea pigs and rabbits! There are tons of adorable photos on the site.
These costumes are all for sale, as long as you can pay with a money order.
Sizing: The rabbit shown is an adult Holland Lop, but the costumes are adjustable for a slightly larger or smaller breed. The guinea pig models wearing the costumes in my photos are full grown (approx. measurements: 8-9″ from back of ears to their bottom, 10-11″ waist) that will help you judge the size of the costumes. Most costumes will fit an average size adult guinea pig, but measure your guinea pig first to be sure it will be a proper fit. If the guinea pig is too young/small, they will walk out of the costumes.
[via Cuddly Cavies]