I personally got several cockrings. (Have been meaning to do a thread on them ...)
Anyhow, Stillwantmore's tip is spot on in my opinion. Silicone ones are good. Definitely look for some width in the cockring.
After a
Bathmate session I usually wear a halved Doc Johnson cockcage ring. Mine have lasted quite well.
Other good option is Tantus Super Soft C Ring, also written about on this forum.
I would like to try the Horseshoe cockring, but so far the cost of one has put me down. However, on the pricier range I acquired myself Jnaja by Velv'Or. It's a whole package deal cockring.
There's one problem that's the same with Horseshoe cockring; the sizing. So, get your measurements right if you go for either one of those. (Or cockrings that are metal / non-stretching material.)
Oxballs also offers some nice cockrings, like Powerballs for example.
Lots of choices out there. The O-rings work as well, if you're really on a budget. I actually bought a box of different sized ones. With O-rings, as 2slow mentioned, width is to be taken into consideration. Most of O-rings available are on the thinner side, so they're a bit rough while they do work. Hairbands as mentioned, also work, just be sure to get one without a metal "hook" in the band. That rule should go for all hairbands though, that metal in the bands does damage to the hair.