You probably don't want me mention the free one you missed out in here yesterday then...
I would have thought some of the traders would have had a dynamo on the shelf. (Paddocks £64 inc, Holden and Auto Electrical Supplies also list them)
There is also the company that always goes to Stoneleigh and has a trailer full of recon ones for sale. Perhaps someone can remember his name.
Is now the time to do an alternator conversion perhaps?