The question is not so much where but more how can I buy a decent one. Look in Loot, Autotrader, Exchange and Mart or eBay and you will find a regular suply of cars for restoration, usually between £500 and £1500, that have gone from one owner who was going to do it up to another to another. Usually botched repair jobs, will cost a fortune to do properly. Club adverts for decent ones between £3,500 and £8,000, but dealers charge more for usually less car worth. Beware of electrical trouble (window motors and sunroof) and especially rust!!!! Even well looked after examples can have the surface bubble and water come up or look inside the front wheel arches, glove box and under the plastic sill cover and be amazed what rot can hide there, so unless you are a panel beater yourself expect a bare metal respray for a 30 year old car. Pre-1973 cars are better, because they have slightly stiffer suspension. Mechanics are very strong. I am currently having my CSL repsrayed, but it still needed two complete rear panels. Once you have a good one you will enjoy her because no BMW looks as good or drives as smoothly.