Welcome to the forum John; was that the eBay one for sale in London?
Bad luck with the tank. I have never seen a Civile tank for sale, anywhere, and they will certainly be far less common than the ubiquitous Militare versions. All is not lost if you can't find one. Tonya's Militare tank was leaking and he took it to his local radiator repair shop, where they cut out the bottom, and either welded or brazed up the leaky joints for a fraction of the cost of a new tank. You can buy tank sealant, but a proper rapir is certainly a far better option.
Hopefully you'll get somewhere with that, if a tank doesn't materialise.
I've heard of people fitting a V35 or V50 tank, but don't know how true it is, and it probably won't be straightforward, as the fixings are different I think.
Good luck!