We broke the traveller car on our Marstrøm T on a tuesday, with a regatta coming up the following weekend. Marstrøm sent us the parts we needed in time before the regatta..
I have bought parts from them before also, and had the stuff delivered timely. I would not worry about getting parts from them, but agree that it's not the cheapest gear around. A $300 compass mount is extreme, that is a good reason to shop around. Marstrøms website is very good, so you can find all prices there and compare before you decide.

http://www.marstrom.com/