You can purchase a RTA ready to assemble cabinet from many sources on the internet. If you purchase a bathroom vanity assembly, the depth measurement will ususally be 21" instead of a kitchen cabinet that's 24". These cabinets are usually plywood instead of high density fiberboard, and are therefore stronger and lighter than those usually found at big box stores.

Having it disassembled from the start makes it easy to modify. The internet sales are also generally less expensive, with some dealers even offering free or reduced shipping costs.