You might want to check Yahoo Japan.  They
normally have some there.  I've seen the prices
range from around $100 to well over 1k.  I've
actually seen a lot of good priced cels there, but
for that you will have to be patient.

Here's the link to YJ for Mononoke / Miyazaki :
http://list5.auctions.yahoo.co.jp/jp/2084035780-category-leaf.html?at=true&alocale=0jp

They have one on there now but it's high priced. 
If you aren't sure how to bid you will have to use
a deputy service.