If you're a professional, the new or rebuilt cameras are for you. These come with guarantee, and are in perfect condition. Of course, this comes at a price that rules out amateurs, ranging from 1200 € up to $ 2200. Have a look at the links page for adresses.
In between you have the option to go for a used camera from a dealer, who guarantees it's function. I'd steer you away from regular photo dealers, since they have no way of properly testing and evaluating a super8 camera. A better place to shop are dealers specialized in film. Anyway, you can be sure that it runs and you'll be able to film right away. Prices are about 500 €, depending on the model and lens.
Cheapest way to get a used beaulieu are, of course, auction sites. I presume that is what most people will do, and indeed this is a very good source. Add the cost of a service, and you end up cheaper than when buying a used one from a dealer, and have a better camera. But don't be fooled to think that you'll get away with the auction price. Either you invest money, or time and (less) money when you want to DIY before you can pop in your first super8 cartridge. Prices range between 50 € and sometimes frantic bidding wars go as high as 300 €, without any apparent logic or relation to the model, lens or state of the camera.