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.
Think you're advanced? Maybe you're not. And that's a good thing. See why Starting Strength author Mark Rippetoe thinks you may be able to build 60 pounds of mass this year. Check it out.