Sporty's Pilot Shop has them,
Sporty's Trifold Kneeboard US$32.50 !!!! Everything Sporty's has is expensive.
I made my own from a fluorescent yellow (so I can see it & remember to take it out of the aircraft) plastic clipboard I got at WalMart & cut down to size, & some elastic & Fastex buckles. The clipboards should be available at office supply stores. Works great, less expensive. I added a clip to hold a pen, which was the most expensive part of the whole thing.