Hello,
As said genuine G40s are thin on the ground. Apparently there are only 81 taxed examples in the UK just now with 190 SORN. I'd assume a good percentage of the SORN'd ones have been robbed of their engines and the shells are just rotting away somewhere.
They do come up for sale though, but if you have your heart set on a genuine one then do your homework when looking at a car. There have been a few recently that have turned out to be conversions, they may try and blag them off as a 'reshell' though.
Cheap ones are none existent unless you get lucky and find one in a barn owned buy someone who doesn't have the internet. Otherwise be prepared to part with upwards of £3k for a decent example.