A variety of places have them, hardware stores, parts stores, HF, etc. Just be sure to check weight rating and what width tubing it goes on. You might also check bolt location so you don't have to drill new holes to bolt it onto the trailer tongue.
Make sure you check the quality. I bought a couple from Princess Auto (Canadain version of HF) for my yard trailer, only used it two of three times and the latch broke right below where the eye for the lever attaches. It was a piece of Chinese junk, would have hated to use it on something going down the road..........Mike
Suggest you look at specialty sites like e-trailer to get quality parts with at least some sort of guarantee/warranty... If using trailer on road, you don't want something junk that will fail at speed..... Trailer couplers are something not to cheap out on....