(a) We are not aware of a "natural"
bedikah cloth supplier.
(b) It is fully permissible for you to make your own
bedikah cloths. Natural cotton is fine (though if you have a choice, the whiter, the better). It should be large enough to cover the finger and reach all the interior surfaces of the vaginal canal, but not so big that it is hard to manipulate. (About three inches square is typical.)