We use Frontline Plus, too. It is available at most vet offices , pet stores, probably Walmart, etc. Kensie, was it really cheaper at pet meds online? Last I checked it was about the same price in our stores so I never ordered it.
I personally don't use any Hartz products, I don 't think they work well and I think (don't quote me) they don't get good reviews.
We we had horrible flea infestations last year. Three cats and two dogs just kept it going almost all summer. What worked for us in addition to the Frontline on ALL the critters: dogs bathed once a week (flea soap or Pert shampoo); vacuum every day; wash all bedding, blankets, pillows, toys in hot water--- anything they were likely to have laid on especially. I bought a flea spray and sprayed each room/carpet/furniture once a week-- keep animals out until it dries. Sprinkled flea powder on the carpets and let it sit-- vacuum thoroughly after time limit suggested on pkg. I spray my yard with Bayer flea indoor/outdoor spray which is good for other bugs, too. It may be over - kill, but two of our animals are highly allergic so it is still cheaper than vet bills for them.
Y/N.... Comb often and keep an eye out for more. I keep a cup of soapy water that I dip the comb in as I work through. Pay particular attention around the tail area and neck/ears. Fleas die in soapy water. Hopefully you caught this quickly and won't need all the drastic measures I went through.
Good luck!