Ace Hardware sells a universal fluid that replaces Hy-Guard (names it on the label even) if you can't find anything else. Lucas make a replacement as well, but locally it may be hard to come by. I'm sure Tractor Supply Store would have an alternative as well.

Here's a link to the TS gear oil... It doesn't say what it replaces but someone commented on it and Hy-Guard is in the list.
Traveller® Universal Tractor Trans/Hydraulic Fluid, 2 gal. - 0806383 | Tractor Supply Company