I have heard some people use Mylar D bags.  Im 
not sure where to get these, but apparently 
they're better than standard cel bags.  They're 
sturdier.  BUT, I also heard the problem with 
them is they create a lot of static 
electricity...so when you take your cels out of 
the portfolio...look for a zap or two.  This is 
why I dont get them...I have a really strange 
problem with static.  I can touch a concrete 
building on a hot day and still get shocked!