Big Dee’s has it: https://www.bigdweb.com/search.do?query=fly+bye. And Amazon has the gallon.
And here’s the mfr: https://guaranteedhorseproducts.com/.
pony_baloney, I apply it all over, but I dilute it a little because it is SO oily. About 1 part water to 4 parts Fly Bye. I tried 50/50 dilution and it didn’t work as well on the flies. And yes, it smells wonderful.