I usually do a Google or Bizrate search and compare prices - they
differ quite a bit!  Last year, I bought from a Canadian site -forget
the name right now.  I rescue & place cats (only have 5 right now, but
Spring and orphaned kittens are coming very soon.)

Just checked out 2 sites.  Fleastopper.com has a 6 pack of purple
(large cats) for $39.99.  Petshed.com has the purple 6 pack for
$31.95, and the orange (<10 lbs.) for $29.95.  That is just 2 sites -
I check a dozen or so, and try to get ratings on the site, before
buying.

A vet tech from our local low-cost spay/neuter service said that the
Advantage formula is the same for cats and dogs.  At her shelter, they
buy the large dog packs & draw off cat doses through a syringe (but
DON'T inject the cat!  That's just to measure accurately.) I haven't
tried that yet.

Here in NJ, I only need to treat the cats for 6 months - April thru
Oct.
Rosemarie
P.S. Fix yer pets - PLEASE