Norton Sales [1] used to be a legendary source of surplus rocket and aviation parts, but these days they seem to mostly rent stuff out.
There is a story how Peter Beck bought there an LR101 vernier thruster from Atlas ICBM, and then smuggled it to New Zealand. Examining the engine showed him that "professional" stuff was not necessarily too much more complicated than what he could make at home. This was probably one of the factors which inspired him to start Rocket Lab, which he did after returning from the USA.