It's just a standard transistor output transformer, you can get them from https://www.maplin.co.uk in the UK, search for part number LB14Q. You should be able to find a German supplier.
It's fine, the transformer (as is the one in the article) is designed to feed a speaker from a push-pull transistor amplifier, so the primary is high impedance, the secondary is low impedance. The project simply uses it backwards.