I know animegame incorrectly describes Pretty 
Sammy Humanoid Ryo-Ohki as Ken-Ohki.  OVA 
Humanoid Ryo-Ohki goes for a lot of $$$.  I sold 
mine on eBay for close to $100 even though the 
amount of paint on the cel was around 2" wide.

Haruna and Hotsuma cels have been appearing a lot 
on eBay lately.  They've been in the $50-100 
range.  Nude Harunas go for $100-200 on YHJ.

Finding nice Achika cels are like impossible.  
Taro's last update had an Achika cel for $300 and 
it sold out right away.  (Close-up of Kain 
pulling her hair).  I can't remember if they 
still have their $450 Achika cel with Ayeka.

As for Kagato, you should be more specific.  OVA 
or TV Kagato.  For some reason, OVA Kagato cels 
are pretty expensive.  Mine with the book cel BG 
cost a little under a $100 from Animechaos, 
though.  (It was part of one of the Tenchi sets 
from YHJ).

For a lot of those people you mentioned, you 
could try visiting Art Shop Rose, especially when 
they update.