I just use an old fashioned Trojan condom. They're a little stiffer and stronger than some of the other ones, so they don't unravel. I take it out of the package and bite a tiny hole in the tip with my teeth. Then just stretch it out like a rubber band to the right size and put your cock through the hole. Sometimes I just put it on at the base of my cock. Sometimes I use two: one around the base of my cock and one around my balls. While this makes me extremely swollen, hard, and makes my nuts hang really nice for fondling, I find that it constricts the power of my ejaculation of cum. The orgasm is intense but the sperm usually follows after. I like to see it cumming out during the orgasm instead of afterward so I usually pull the one off my cock right before I cum.
What I've found works best for me though, is to just use one condom ring and stretch it enough to go around my cock AND my balls. This way the tube that the sperm travels up is not constricted but there's more than enough constriction to slow the blood flow into my cock and cause a hellish boner with a blazing red head.
I do like the ideas of O-rings that I read about above. These would be reusable. The condoms generally are only good for one or two sessions. However, O-rings are pretty hard to stretch, so the size needed to get them around your dick and balls may be to loose. Another thing, if you are able to stretch one enough to get it on, it may be too tight, and you don’t want that! I guess I'm going to Home Depot! It won’t hurt to give it a try but might be a little tricky pulling out my cock, pumping it up to size and testing for a good fit. A plus though, is that usually a store clerk will come up to you and ask if you need any help! Won’t they be surprised when you turn around and say, “Sure!”