They don't sell any polypropylene/mylar 
envelopes.  (that is what you would search for; 
avoid polyethylene)  These envelopes should be 
clear, about 2-3mm thick, and have a flap (with 
or without adhesive) at one end.

I prefer open-ended envelopes myself (no flaps, 
open on both sides).  I get mine at this site:

http://www.iconusa.com/Onlinestore/sleeving.html
(See S19050)