I don't think you'd be going too wrong if you 
got the Heiko bags.  Picked mine up in Japan a 
while back at Anime World Star (large cel 
dealer), and it seems like they're the type most 
Japanese collectors use.  Clarity is excellent, 
noticed some bag warpage when in direct contact 
with cel paint, but I think that's normal.  $11 
for a bag of 100 seems reasonable as they cost 
roughly 800-1000 yen in Japan.