Just noticed it this morning.  IconUSA has 
archival matting/sleeve combos for sale now as 
well as archival preservation cases to store the 
matted/sleeved cels.  (url above) They have a YUA 
repro on the page as an example.  Obviously, 
they're more pricy than bags but well worth it 
for your expensive and/or very emotionally 
attached cels.

Later, ya.