Here is the current low budget fly spray system by Country. You can find it at TSC and most farm stores. Can cost you up to $50. The fly spray canister on its own is about $8
Here is the "home" air freshener automatic dispenser. Lysol, Glade, Airwick... they all make a version of this and are all interchangeable. Dollar store versions run as cheap as $5.
What you need:
1 "home" automatic air freshener dispenser.
1 air freshener canister
1 Country fly spray canister
Take the nozzle off of the air freshener canister. Replace the fly spray canister's nozzle with the one from the air freshener. These remove by simply pulling straight up. To replace, push nozzle down onto tube. Don't spray yourself :)
Insert fly spray canister (with new nozzle) into air freshener dispenser. Set the dispenser settings to your preference and insert batteries. Hang it up high in stall (try to keep it away from eye level with horse) and you are all set!