I use
http://www.omahavaccine.com cheaper than most stores even factoring in the shipping. The promotion this month is free shipping with a $50 order. I only had 1 order messed up in the 6 or so yrs I've been using them. They took care of it right away. They also ship to APO/FPO boxes which is nice.
ps the shipping promotion is until dec.. oops.
to get the heartworm and other prescriptions meds all you need is the script from the vet, which most will give, and fax it to them.