I don't know about Boston, but I got mine at a place here in Chicago called Sensaphonics. As far as I know its the only place that does everything in-house. I just made an appointment to get the molds taken and came back to pick up my earbuds a couple weeks later. They ran me about 175$ for the earbuds themselves, and another 175$ for the hearing test, impressions, molded sleeves, etc.
I would *strongly* recommend going to an audiologist to get the impressions done. Then you can send these off to anywhere you want to get the molds made. Impressions involve having a piece of foam placed very deep into your ear canal. This process can be very dangerous if you don't know what you are doing, plus its a rather uncomfortable feeling. I can't imagine trying to do it myself. If you mess it up, you can damage your eardrums or end up in the emergency room to get the foam stopper back out. At the very least, its highly unlikely that the impressions you make yourself with get you a well-fitted earbud. People go through (at the very least) a good amount of training to do this, and for good reason.
I know its tempting to save 175$ by doing your own impressions, but its sooo not worth it. I can pretty much guarantee that the results will be unsatisfactory, and you could hurt yourself doing it. Plus its worth having a full-on hearing exam (not just the 2 minute, "tell me when you hear the beep" stuff they do at a doctor's office or whathaveyou). They were able to point out weak and strong frequency ranges in my hearing that has helped me immensely in my recording and mixing jobs.