I don't know if you can buy the pass the day of. I think you have to prepurchase it through boys and girls club (I think that's who's hosting it).
As far as buses go, you can go to the nearest park, and then get on a bus to Saratoga Springs and walk from there. You could also look into the DTD resort buses to pick up at one of the parks. Or you could go to the TTC and hop on the Lynx bus, 111 or 50 bus goes through DTD. Both of those start running at like 6am.