Fill it at a normal pharmacy like Kroger or Walmart. They fill pet prescriptions at a much cheaper rate than at the vet's office. I never obtain antibiotics or medicines directly from my vet - have always found it cheaper from the source. Get a prescription and fill it at whichever pharmacy gives you the cheapest deal.
150$ for two weeks worth of medicine that can be bought on Walmart's 4$ medication plan is crazy! Do not resort to sources outside the country - I would not trust them if I were you.
You can also get medicines from online sources like 1800petmeds and similar sites.