Dragon Man has that covered for you really, on the mk2 the 4x4 is metal and the 2wd is plastic, the carb and injection ones differ too on those I believe, the UK only got the carb tank of course for the 4x4 but if you buy a tank overseas for a 4x4 be careful it isn't an injection one as the hole is different.
If you are still looking for a tank then Benjamin in the Facebook group "Fiat Panda Classic Owners Group UK" is giving one away that needs repair that needs collecting from Angmering, which I know is a long way from you, but if you reason with him and offer him a suitable cost he may be able to post it for you... worth a try?