It was done with cels. However, they are not 
common (production on the series was done over 4 
years and only certain of the OVA's seem to have 
cels released from them), can almost only be found 
in Japan, and the prices on them are already high 
due to the popularity of the series in Japan. Once 
the show makes it to Cartoon network, I have a 
feeling I will not be buying new cels from this 
seris.

Catch you later,
Marcus