I've never found a place cheaper than
www.petsuppliesnet.com. Here's a
link to a large thread on the website. That's where I heard about it.
They're very legit and based out of Australia. Best of all they don't require a prescription for heartworm preventative. I order HeartGuard and Frontline Plus from them about once a year. The only knock against them is that it takes about 2 weeks to receive the meds.
Here's a word of advice. If you own more than one dog, then buy a dose for a larger dog and split it between the two. I split a HeartGuard for 26-50-lb dogs in half for my two under-25-lb dogs. I also split the largest size of Frontline Plus among all three of my dogs. I save boatloads of money by doing this.
Edit: It's funny; I just now read your OP. I would bet that petsuppliesnet is the Australian company you're thinking of. Initially I just read the subject and hurriedly posted about the uber-cheap site.