Ask at your local Trek shop or similar brand bike shop. They sold aero bars with 26mm shims for their 31.8 bars which were generally discarded. Or there are certainly Problem Solvers shims available.

However, you should really consider why you want them on your bike, outside of free take-offs. For comfort? Bzzt -- get your bike fitted. For speed/aero? Bzzt -- you ride a single-speed and are very often not operating at optimum efficiency where aero bars would boost that.

Sell them off on CL and invest than cash elsewhere.