Rubber "O" rings are available at Home Depot in graduating sizes, and are pennies on the dollar. Buy all the inbetween sizes from 1.75" to 2.5" and see what feels good. If you get it pulled on and you freak out with a hardon you can't control, you can easily take a scissors and carefully cut it off. Can't do that with metal. Tell any nosy clerks your daughter has a girl scout project.
Keep track of which size works for you best. Put your dick thru, then add one ball, then the last one. It's a bit uncomfortable of a pull to get it situated, but you'll soon get the hang of it. Some guys put both their balls thru and then their flaccid dick- I personally think that works a LOT better myself, and is less jostling of the boys. That should help to bolster your hardon when you need it. Note the rubber can pull on an ungroomed patch of pubic hair.
Once you get your size you can invest in a simple metal ring- look for one where the weld (where the two ends are joined to a circle) is TOTALLY polished away inside and out. These double nicely as hand-towel rings for your bathroom. A properly sized ring should allow your pinky finger inside the ring with all the junk as well... Again it's going to be a learning curve to get used to wearing it. Feels very different than rubber- and when you get hard you might panic if it's a bit too restricting- that's why sizing with a rubber ring first is crucial... remember when you nut it WILL go down and then you can slip the ring off.
Once you have a general idea of your size and a bit of experience wearing them occasionally (NOT ALL DAY!) then you can move on to a cockring manufacturer like gear essentials or fort troff and invest in something better- around $30-45. Some people say that if you wear them all day it can affect overall sensitivity- like the way that bike racers can experience saddle numbness. Others have no issues at all.
Note a HEAVY thick ring (donut shape) looks impressive and the weight feels wild, but again in limits your availability to wear it long time- go lighter until you know if that weight might make your nuts ache like a severe case of blue balls. Pouch underwear is quite recommended- because too big of a ring can slip off of a deflated cock, go down the pant leg, and roll along the floor of your kid's school hallway much to your chagrin
Another good version to have around is a "ball stretcher." It's like a cock ring, but instead of a metal or rubber ring, it's a flat piece of finished leather with a couple snaps on it. You wrap it around the base of your balls, behind the cock, and close it with the snaps according to how tight you want it- like a toilet paper tube with your junk pulled out one end, balls and all. Ball stretchers come in widths from 1" to 4"- the WIDER the size, the longer it is, and the farther your balls are stretched down (or away) from the body. The widest are for more advanced stretchers- and the wide sizes (3 or 4") can't be worn for long periods of time like a lesser width. A stretcher will give you 'low hangers,' plenty of pictures on this site.