Â£1000 is a bit extreme, I would expect around Â£250
Narrow-boat people have the same problem and they can get 12v and 24v alternators to drive from low speed diesel engines running from 200 to 1200rpm.
A quick search on Ebay and other forums gave the following potential suppliers;
ATP Electronic Developments Ltd. Cannock 01543427900
Ed Shiers Stafford 07922 163072
G E Middleton in City Road, Manchester 0161 872 8409
Or you could try this DIY guide to build a permanent magnet alternator;