There are so many on the market.
Anyway, ROP isn't suitable for cock ring usage, at least not when you seek hard clamping power. Sure you could use it together with a cock ring, no problem in that.
A basic adjustable cock ring is your best bet. Silicone is a good material. I advice to stay away from stretchy TPR or the kind, I've broken those so many times. Only good for few uses, in my opinion. And they're usually not the most fit kind, it's difficult to adjust the power of clamping.
Check your local adult store or surf the web: most of the shops are bound to have few adjustable rings and the price is usually under 10 USD.
So, my recommedation: Adjustable cock ring. Or then a buy a set of different sized silicone rings, they usually go for 10-15 USD.