Bayer, the company that sells Advantage, a good and safe drug for
controlling fleas had a problem.

The problem is this -- they make and sell the drug in only one
concentration -- 100g/litre or 9.1% concentration.  Much more of the
drug is needed for big dogs than for small cats.  A big dog needs 10x
as much as a small cat -- but big dog owners aren't going to pay 10x
more than small cats owners.  Put another way, Bayer isn't going to
sell a 4 or 6 month supply of the drug for a small cat at 1/10th the
price for a large dog.

The Bayer solution:  Divide up cats and dogs by weight and sell
Advantage in different packages for each size and for cats and dogs.
Inside each package are 4 or 6 tubes of the drug at the correct amount
for each size animal.  That way you can sell Advantage at about the
same price per cat or dog and make much more money.

Currently there are 6 sizes of Advantage available, all with the same
concentration  of drug.

Cat owners can take advantage of this.  Buy dog size and use for your
cat.   I bought the 4 month medium size for dogs.  A  4 month supply
for a medium dog is 4 tubes of 2.5ml or 10ml.  Our fat cat uses 0.8ml
per months.  So I get 10ml/0.8ml or 12.5 months supply for the price
of 4 months.  

There's a web page that goes through the details (especially mixing
the drug: http://www.lisaviolet.com/cathouse/advantage.html   And for
a detailed version of what's in each color of Advantage - see the
bottom of the petshed web page ( see below).

The web page suggests using a syringe to measure the drug. If you have
trouble getting a syringe (even without a needle),  you might find it
easier to  go to a pharmacy and ask for a 5ml or 10 ml Exacta-Med oral
drug dispenser ( or something similar). Interestingly enough, I got my
5ml size one with a custom made cream for the cat. This simple
dispenser  makes it very easy to measure out the exact amount you need
-- either 0.4ml or 0.8ml and store the rest in the dispenser. If you
go a bit over, that's not a problem -- there's a large safety margin.
See http://www.nofleas.com/FAQ.asp#q9

As for price, you can certainly buy Advantage a lot cheaper if you
don't buy it from a vet. As far as I can tell, the cheapest place to
get the stuff is from Australia.   I'm in Canada and I've just ordered
it for the 2nd time from Australia.  Unfortunately, some Australian
companies will not ship to the U.S.  

If you buy from an Australian company, make sure that you compare
prices in similar currencies.

Here's one company  that won't sell Advantage to the US:
http://www.pets-megastore.com.au/index.php?currency=USD&cPath=2_12&sort=2a
(look under dogs for the larger size tubes).
This is the company I've ordered from twice.  They have a reasonable
shipping and handling fee.

Here's one that appears to ship to the US:
http://www.petshed.com/advantage.asp   - If you're in the US and order
from this company, make sure you read the disclaimer at the bottom of
the web page.   I have no idea if US customs will stop the import of
Advantage into the US.

petsmegastore will sell Frontline Plus into the US.  As to using the
dog versions of Frontline Plus for cats -- it will be safe, but may be
less effective.   If you look at the bottom of
http://www.petshed.com/products/category338.asp
you'll see there's a lower concentration of one of the drugs
(Methoprene) in the dog version as compared to the cat version.

Hope this helps.