Hi Giles,
Welcome to the forum
As above the best place to find a C5 is eBay, but the problem is they sometimes need a little work to get them going. The C5 Wiring kit is a great solution as it opens your choice for many different types and sizes of batteries, it also has cross wire protection !!
Keeping the C5 stock is one option, but as you have found out as the parts run out - the price goes up. The ethos of C5alive is to keep the vehicle on the road & have developed (kind) mods so that your C5 can be reversed back to stock when needed. I am a keep it original guy, but also a realist and so I bought two - one totally stock with original stickers key block - battery the lot, the other with just the bare essential mods to keep it usable - Wiring Kit, motor fan ETC.
Adam Harpers price also reflect that they are the original spare parts, stored for 25 years. Many C5’s would not be working without him and he’s a top bloke too having met him at several events, where he gives a bit of a discount.
C5alive also has quite a few second hand parts for sale, and as a community there is always someone willing to sell parts to other members.
I hope you enjoy the forum and hope to see you both at some of the up coming meeting,..
Regards
Ian