I've bought a Wacom Bamboo tablet for it. It cost about 50 Euros. I've read that in China you can get writing tablets that work just as well for about 20-30 US-$. And then it actually trains your muscle memory.

But I don't think you actually need it. You can make your own offline writing sheets with the HSK vocab list and Excel, print them and then just use those to practice writing.Test