Plenty of antennas out there depending on what you want to listen to. The RTL-SDR coverage is generally from 25 MHz - 1700 MHz which is a wide range. I use an outdoor discone mounted on the roof which is a decent wide range antenna. Go to
https://www.rtl-sdr.com/ and read through the postings about antennas. Whether to build or buy is up to you, depending on your skills. Outside antennas outperform indoor ones in most cases. Good luck and have fun.