Cels Forum

Almost all of this has been said before but -
Buy cel books made of polypropylene. For example, two types of "ITOYA Art Profolio" work:
IA-12-11 having 24 pages 11" x 14"
IA-12-14 having 24 pages 14" x 17".
They also have ones with 48 pages but I would advise against using them, since they get heavy & bulky when filled.
ITOYA phone (800)628-4811 / (888) 588-0345 (Canada)
Web Site:http://www.itoya.com/main.html

I put cels in polypropylene "music sheet size" bags available at comic book stores (about $8 per 100) and place a sheet of Aquabee No. 522 Glassine Tracing Paper (it's labelled Neutral pH, which is the same as acid-free) between the painted side of the cel and the bag. (20yds x 36" roll costs $12 per roll.)  I have collected several samples of "silk paper" from various Japanese cel dealers.  The "silk papers" range from the wax paper like Glassine Tracing Paper, which I prefer, to the Kleenix type tissue paper.  The tissue paper tends to stick but rips off easily.  I've been happy with the tracing paper. It does not stick.  To stiffen the pages of the Itoya portfolios, I put a acid-free backing board in the portfolio page pocket with the bagged cel (but not in the cel bag itself).  An example is Collector Safe brand Magazine Backing Boards ($8 per 100), available at comic book stores.  These do not fill the sheet pocket up (you could use 2 and tape them together to completely fill the pocket - watch the tape, though).  But I've had good success with just one in the pocket. (I use these because they are available cheap.  The acid-free stuff at art stores tends to cost more.)

If you have the cels in the bags, then it doesn't matter what the insert sheet of paper in the pocket page is, since the paper won't be in direct contact with the cel.