I have a hydroboost on a vehicle and never wondered about rebuilding the unit until your question.
I looked around the web and found info about where I expected to...mainly on the diesel forums, like this one:
That link tells you how to removed the accumulator, but not where to get a new one. I'd imagine a bit of searching should turn up a canister.
A brake shop probably could do the job, but then so could any other shop. Looks to be pretty straight forward mechanical stuff.
Other links I read leaned toward a replacement rather than a repair. I don't know this to be a nearly universal truth, but since we tend to be the "If it's broke, we fix it." types, a repair is worth exploring.