Herbie is right, Ardusimple has some good antennas. I have the basic survey antenna I am using for an AgopenGps project.
One thing to be aware of is the Ez guide 250 supply’s 5 volts to the patch antenna and 12 volts for the AG15. The ArduSimple survey antenna runs on 3-5 volts, so if you go that route, make sure you select patch antenna in your Ez guide 250 so you don’t send too much voltage.
I have a spare brand new Ardusimple patch antenna that should work with your Ez guide 250 if you wanted something cheap to make it work.