Do any of the cels have sketches?  Someone had
mentioned recently about taking the sketch (maybe
scanning it to edit off some markings) and getting
it printed on acetate making a correction layer. 
I thought this was a good idea, I have several
cels that I would like to do this to.  

You also could take a scan of your cel and edit
out the color leaving the trace lines.  Seems very
time consuming to me but I think it would be worth
it in the end ;)

I also read that you have to make sure that the
acetate is the kind that can be printed on.  A
good place I have found that sells some that will
work for laser copies is www.dickblick.com.  They
have Dura-Lar:

http://www.dickblick.com/zz555/06/products.asp?param=0&ig_id=1597

I hope this helps, I know it's not really what you
asked for but it might be cheaper with pretty much
the same quality as getting it professionally done
and it is not permanent so you don't have to risk
damaging your cel.

Take care,
nesuferit