FA sells a couple of antennas, see ads-b-flight-tracking-f21/new-inexpensive-1090mhz-antenna-for-sale-t35561.html (it’s out of stock right now, though I hear that more are on their way to amazon now); that’s probably one of the cheaper options for an outdoor preassembled antenna.
Or there are a whole bunch of ideas for different DIY antennas here (and elsewhere in the ADS-B forum): ads-b-flight-tracking-f21/three-easy-diy-antennas-for-beginners-t20177.html. You may have to get creative with the weatherproofing 
You will get much better results if you can put the antenna outdoors with an unobstructed view of the sky. ADS-B signals really don’t like walls / windows / roofs much…