For beginners and stab training, we just used washable colored markers and white tee shirts. It is cheep and gets the point across.
We train in multiples of 10.
10 times slow to demonstrate the technique
100 times medium to feel the motion and correct
1000 times to train the muscle memory
For advanced training, we use aluminum training knives from Steve Rollert (www.doveknives.com) and “ink” the blade edge as well. Steve is a professional knife maker, does custom orders, is a martial artist as well and a licensed EMT. Good all round guy. His commercial web page is http://www.keenedgeknives.com/KE-Practice.html if you are interested in something.
http://www.pabujinkanbarndojo.com/equipment/index.asp