Cels Forum

On the subject of Miaka cels 
Nuriko's cel shop has a few nice ones also check out ebay
Item #326472570. I don't recognise the seller though.

For cels from X/1999 good luck! Best bet would be the Japanese cel shops (refer cels.org) with a quite few good cels of Kamui or Konoe. 

I've personally despaired of getting a nice one of Hinoto despite the murderous prices I'm prepared to pay for one (rather like a good cel of Juri Arisugawa from Utena).