For your practice

Here you can find a web-based trainer by Stefan Kernen

https://webapp.rechentrainer.ch

Here you can generate exercises for mental with solutions yourself and then print them out in pdf format  (only in German) 

Mathelust is the website of Martin, a former participant and trainer in mental math competitions. It teaches  useful calculation techniques and much more in German German.

Schabi offers mental math exercises at different levels in German.

SIKORE is another online generator for mental calculation exercises  in German.

Calculation test is another online generator of exercises in  German.

http://www.math4me.de/ offers exercises for beginners and advanced in German.

Math Trainer offerst tasks and exercises for beginners and advanced  human calculators in English.

World Mental Calculation offers additional material and links especially  for advanced human calculators in English.