It depends on what you are looking for. Parts specific to your caravan will normally only be available through that company and their agents, for parts Bailey Prima Leisure and Bailey agents. Most caravan workshops can get the parts if they need them for work they are doing.
General parts for Dometic, ALKO and the like normally through dealers, accessory shops and Amazon. We have found Prima Leisure good on general things and CAMH members get a discount on many items. In practice there are many good suppliers rather than just one.