It's usually available in sporting goods stores in the hunting section. Some farm supply stores
sell it. A search for "coyote urine" on amazon returned a number of different options. So it's pretty available.
Do you have a rooster? Sometimes an aggressive rooster will take on a predator. Mine do battle with hawks and falcons from time to time. Sometimes you have a bloodied hen and and battered rooster but at the end of the day all heads are accounted for.
Also, you don't need a big dog to scare a fox. A little yippy dog with attitude will do the job. Probably. Much more mum friendly too