A 12 x 12 rosewood (genuine) would not be an easy find. Luthiers would be all over it if it was readily available. You have a better chance of a 6-7” wide that is 10/4 to 8/4, book-matching it to get that foot print. Though if you are looking for an impostor like morado/Bolivian rose/pau ferro or Santos rswd. or the like- those species tend to yield wider material from said logs. Having said that, well I forgot about the most commercialized rosewood that you might find in that size that is slab sawn, East Indian rosewood (dalbergia latifolia).