Some things are just harder to insure than others.
Take the classic example of MGM taking out a 
million dollar insurance policy for Judy Garland's 
singing voice. It mostly was publicity, but it 
still is a legitimate policy. Anything is 
insurable at any amount, but the cost may be 
outragous.

In regards to cels, they just have to be insured 
under the correct type of policy. Homeowner and 
renter policies will not cover them, since they 
only will replace items at their material cost 
(the cost of the acetate). Cels fall under the 
category of artwork in regards to insurance 
policies and a seperate art or collectable rider 
needs to bought in order to insure your cels. This 
place I think pretty much gives the lowest rate 
for collectable insurance, but I do not know how 
good they are when a claim is filed:

http://www.collectinsure.com/quote.htm

Hope this provides some help,
Marcus