Tri-Van truck body is near me in Ferndale, WA - they do a lot of custom builds for all sorts of industry (ambo's, service trucks, hot-shot crew transports etc). I took a tour of their facility as part of a project I'm proposing at work - it's a full service facility and they do really nice work.
that said, while mine isn't a budget build, i'm hoping to avoid needing a shop for anything since labor is a huge expense