Mary Ann - I don't know where you can buy polycarbonate sheets other than Stampers Best, I only know you can because of this blog post where someone did. May be you could email her.
Just had to do it: Polycarbonate sheets for clear stamp storage
Like I said, I have only Stampers Best.
These: StampersBest CLEAR ClingStamp Storage Card 15 pack for $25.00
Their clear polycarb panels are thick and very rigid. As in, you probably couldn't cut them with your guillotine trimmer, too thick. SB also sells polycarb laminated sheets. I have some of those also, as well as laminated sheets from a couple of other places.