Hmm I feel like this came up recently about the UK, which of course is not quite Europe anymore (different topic but Ugh)
Let’s see.
You probably saw this?
Maybe @Franziska.Jentzsch found some?
European material suppliers - #14 by Franziska.Jentzsch
I googled a bit and found these dudes:
https://signtradesupplies.co.uk/catalogsearch/result/?q=transfer+tape
They have high tack r tape. I might keep looking for medium tack but that’s your call.
Found by googling “sign supplier Europe”
https://www.google.com/search?q=sign+supplier+europe
Anyway that should give you a good start. I might also call local vinyl or sign shops and ask if they sell it.