Proof Rolls..

Proof coins are only found on the secondary market from people who have broken up quantities of proof sets and made rolls of the denominations. This is common for people who want to keep the silver coins, and regain some money by selling the non silver proofs from the sets.
I have seen some from time to time at different online coin stores (Collectons was one), and ebay at times. You just have to be at the right place looking when they go up for sale.

The mint would likely never offer rolls of proof coins simply because that would mean letting the coins touch each other, something that proof coins should not do. Proof coins are individually minted, handled and packaged (although there are now some machines that can do this) by hand. That is part of why even clad proof coins are expensive, metal content becomes a small part of the cost of making them.