Cockrings might be quite expensive. If you just want a cheep simple ring. Go to a hardware store that have equipment for boats and their owners. They often have rings in several sizes to be used for ropes and hooks. Those in the picture is 50 mm in diameter, and costs about 2 € for two at the Biltema store.
Note, you might not know what the ring is made of, so ask, or keep it directly on your skin (not around your cock) for some hours to be sure you don’t have an allergy to the metal (like nickel for instance). Clean the rings thoroughly before use.
If you are going to buy your first cockring, I would recommend to read this post first: Want a cockring?
Cockrings is mostly used to keep your cock hard for a longer time, but for some of the rings are more a part of their gear then for keeping blood in the penis. Maybe fasten to a harness or a jockstrap. In any case it is hot.
If you have never used a cockring before, you should find your size. The easiest way is to use a measuring tape and measure the girth of your cock (while hard) and balls at the root. Take the number and divide it by PI (3.1415) then you get the required inner diameter of your cockring. For instance if you measure 15.7 cm, you will require a cockring with 5 cm in inner diameter.
I recommend buying rubber rings with small thickness in some bigger and smaller sizes. Then you can easily cut it off with scissors if it is to small and not letting the blood back. It is a bit embarrassing going to the emergency room to ask for help to remove a steel ring.
As always there is a lot of different designs and materials. For instance steel, rubber and leather. The ring itself can be thin (3-5 mm) or thick (>10mm). Thick steel rings might also be in different finishes, like brushed and engraved. So you can always find something cool.