Buy a standard wrist roller if you want, but a homemade version of PVC pipe (1.5 to 2 inches is perfect) with climbing webbing attached is a cheap and amazing solution.
Check our power-rack version that we made out of aluminum tubing. The beauty of it in a rack is that you can truly test the grip without the shoulders giving out first.
Dan John wants to know if you're making progress. It seems the answer should be simple, but is it? Fortunately, the big guy offers up 3 unorthodox ways to gauge your answer. (Wait until you read about the "pencil test".)