For the most part, we use good broad spectrum antibiotics in treating parvo cases. This typically consists of an oral sulfa based antibiotic and an inject-able cefalosporin. The antibiotics are primarily used to prevent secondary infection that can creep up since the immune system is severely weakened.
Is this the kind of thing we can find at the farm supply store?
Can we administer it ourselves?
It is possible you can find a sulfa drug or penicillin at a farm supply store, but not real likely. Most all antibiotics will require a prescription. You will have better luck at a vet supply store. Many vets will understand financial constraints and dispense the medication to attempt treatment at home. Though you will probably only be able to get the oral medications.
You're welcome, and good luck.