This is a combination of Metronidazole (250 mg) and Praziquantel (75 mg) per packet. I found it in the fish aisle of the local PetSmart while I was picking up some feeder goldfish for the backyard bond. This is a nice one-two punch for Giardia, and in a perfect one-packet-per day dose for a working terrier. Five to seven days is the normal prescription for Giardia, but a ten day dose will do your terrier no harm, and may do some good. Giardia is mostly self-curing, but it's still good to have this in your home vet kit, and no prescription is needed this way. If you have a number of dogs, consider simply ordering FishZole online as it comes in an easier-to-administer tablet or capsule form.
Another option is Panacur or Fenbendazole, sold without prescription at PetSmart or your local pet or feed store as Safe-guard, and used on-label as a wormer and off-label as a treatment for Giardia. Notice the "zole" at the end of the names of both drugs? Same class of anthelmintics, and both are part of the imidazole family of drugs. Metronidazole (Flagyl) is generally prescribed to humans for Giardia, but Panacur works off-label for dogs (or humans) as well.
Amoxicillin and Cephalexin are also available in the fish section at Amazon, as I have noted in the past, and Doxycycline is available in the bird section.
Get gel caps at a health food store if you want to administer any powdered antibiotic as a pill. See the antibiotic section of terrierman.com for dosage. I do not answer veterinary questions. If you cannot use Google and common sense alone, I cannot (or will not) help.