Most walkons will sell paint by the case, and at a fairish rate. I'd expect to easily get through full case in a days play, but I do play very aggressively and try and spend every game up the front end of it. Most places will not allow you to bring back paint on another day, but they might.
I often find they'll be someone willing to split the cost of a case of paint towards the end of the day if you are running short.