There are some cheap ones I've used that work pretty well. Look for something about 6" long. That should be about a 1/2 wave. If you have a VNA, you can verify it's centered at 915 MHz. HRCC on youtube has a good video showing how to use the NanoVNA. Not a bad piece of kit to have around. Rokland has some proven ones, also. Stock isn't great, but it works. At the end of the day, what you need are more radios in your network. Antenna matters less than that.