A platty sibling is the second half of what you need to recreate the platty daddy. When bred to a normal, the platty daddy produces lesser platinums and normal looking siblings that carry a hidden gene. Breed one of those hidden gene siblings to a lesser and you have a chance at producing a platty daddy.
As far as the availability of them, your best bet would be to get them straight from Ralph Davis. I think someone produced a platty daddy last year on accident, but other than that I haven't heard or seen anything about more being produced.