Pentobarbital, aka Euthasol, aka bluejuice, is a barbiturate and is what is commonly used for small animal euthanasia.

Europe imposed strict export controls on it and other drugs in 2014, but I don't think vetinary supply has been affected.

https://www.bbc.com/news/world-europe-16281016

https://en.m.wikipedia.org/wiki/Phenytoin/pentobarbital