I'll jump in the camp that says you can start with a department store, just make sure whatever you buy fits properly. You seem to have the shoulders down, but don't get anything too long or short, and make sure the pants aren't baggy. Do you have a Macy's within driving distance? I'm sure the SF gods will strike me with lighting today for saying this, but I think you could buy one of the Tommy Hilfiger suit separates and look pretty good. I think those run for $250 or so.
For the next 4 years will will have very little life outside of medicine. You will be seen primarily in scrubs or with a white coat over your trousers. My wife is a doctor, and many of our friends are doctors. I would say only about 10-20% of the guys put any effort whatsoever into their attire. It's a diverse bunch, and they aren't IBankers. They don't care if your suit is fused. Focus on fit and stay away from gaudy patterns, you'll be just fine.
Oh, and take the tag off the sleeve.