Note that Komatex is essentially the same as Sintra, just a different brand name.
I just got my first batch from Ridout Plastics: http://www.rplastics.com/komatex.html
I believe Junkers Cosplay (StabbityBlkMage here on cosplay.com) gets his from San Diego Plastics: http://www.sdplastics.com/komatex.html
I tried my local sign shop that I already have a working business relationship with, but they weren't familiar with the material... I ultimately decided to get a lot more than what I need for the project I'm currently working on, so I have extra for future projects.