Cels Forum

No, Ah My Goddess only has 5 episodes, it is not an on-going
series.  However, there is a mini TV series featuring the
super-deformed AMG characters recently.  Since there are 
only 5 episodes available; of course, there are not many
cels available.  You can still find them, but they're not
cheap except the cels of Keiichi.  
About Vampire Princess Miyu, some dealers still have the
cels from the OAV series.  You will probably find more
later on as they release more cels from the TV series.