For your practice
Here you can find a web-based trainer by Stefan Kernen
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
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.
Calculation offers additional material and links especially for advanced human calculators in English.