Where from? For starters, ask around to see if any one has one, in good working order, and get them to show you the basics. Alternatively go to the Widescreen Centre in London (or Nottingham) or get them to send you their brochure (full of cameras, films & accessories). Beware - there are film formats called Standard, Double and Single 8 - these formats need different cameras. So double check that you're buying a Super 8. See 'Useful Links' for further advice.
The names to look out for are BRAUN NIZO (models 481, 561 and 801 - each of these also comes in a 'macro' model), the BAUER 700 series and CANON (models 514xl and 814xl). There are many other makes but in my experience, these are the most reliable with a good range of functions. Cameras are plentiful on eBay but you must act with caution - buying 'sold as seen' cameras is not to be advised. It's likely that if a seller is selling a camera along with other Super 8 goods then they may be something of a specialist and thus probably more reliable. Always check that the built-in light meter is working and that, if the particular model needs it, that the 'battery case' is not missing. Without these the camera is virtually useless.