as someone who's been on a similar quest for 
some time... i can only suggest keeping your eye 
on yahoo japan (best bet; a few beauties popped 
up recently), mandarake daily auctions (mr. 
k/hiroshi genga set ended today), and private 
collections (which may be put up for offer, 
though seems less than likely when it comes to 
gravi)... and having a reasonably bottomless 
wallet helps. 

get ready for pretty fierce competition with 
this series... especially since only the gravi 
ova was cel-done and the show seems to get more 
and more popular as time goes by. the domestic 
release of the show in the us probably won't 
help collecting matters either. 

also, there was also a (small?) number of repro 
cels released earlier this year, so shooting for 
those (if they pop up on auction) might be a 
more feasible idea. a set of crystal sheets (i 
think they were mini) was also released 
simultaneously, so if you don't want to pay so 
much for cels... the mini crystal sheets show up 
on ebay from time to time.

sorry to sound so pessimistic... i've had the 
worst luck with this series, so i'm not the 
happiest gravi collector to be giving advice. 
but i hope that helps, in any case! ^^;

/t.