...the only cels from Gravi you MIGHT (and I 
stress this point) find are from the OVA.  The 
TV series was CG.  Just like anything that's 
relatively difficult to find, your best bet is 
to try on Yahoo Auctions.  There've been a few 
up there in the past, but... you wouldn't be the 
only one who'd like to collect these cuties, 
which mean the end prices could skyrocket...  
But good luck to you in your hunt!

Rekka