Sulfasalazine is probably the oldest medication for ulcerative colitis. But something like 30% of people are allergic to it, so it can be difficult to tolerate.
How to get a prescription without a doctor's visit? Ediekristen recommendation seems worthwhile to pursue. I would also think that if you explained the situation to a doctor -- perhaps even one of those quickie clinics popping up in drug stores -- you could get a script pretty quickly and the office visit wouldn't be that expensive.
Another thought would be to get a partial fill on the script you have. At this point, you need about a ten-day day supply to get you to 02 July. Ask for it. That would be, what, $100?