I bought a small pint from this source:
Ranger Tire Paint - Welcome
I get the impression that a couple young guys started the business when they stumbled on the fact that road striping paint sticks to everything and doesn't come off easily (they're right). I never did get my Billboards (at least not yet), but I did test this stuff on some other tires. It needs a good 3-4 days to really dry well, but it's very durable and whenever I switch to Billboards someday this will be the first stuff that I give a full beta test on.
-Dean