hi there - 
just so happens that one good source is pretty
much right around the corner from me (lucky, ain't
I).  The link above is there website.  They have
both bags with and without adheasive, as well as a
multitude of bag sizes.  However, they are NOT cel
savvy.  You will need to let them know what size
bag you are looking for (the ones I use are 245 x
300 mm for cels and 260 x 400 mm for backgrounds,
but your milage may vary).  And they do mail order
too.  You'll have to buy them in bags of 100
however, as that's the way they sell them.  

Hope this helps...

buma-