Brass fishing swivel. If you like it, you can buy them by the baggie at the sporting goods store. Choose your size. Cheap. They are usually tied between your main line and leader, to prevent line twist, FWIW.
1st - I noticed this was your very first post - so, Welcome Aboard Codycal555! You didn't list your state (or country) in your profile. So, you might consider jumping over to Sub-Forum: Select Your Area.... for information (i.e., clubs, hunts, finds, legends, maps, etc.) directly related to your state (or country).