If you absolutely cant find a 36", the next size down would be 24", but that would only cover half your tank. If your beardie spends most his time on one side, you could put it towards that side to make sure hes getting enough uvb. You want to make sureits on the basking side though, next to the basking bulb.
Have you looked on amazon.com, ebay or lightsofamerica.com?
And have you checked hardware stores?
I know you can get one from petmountain.com, but its a reptile fixture, so its a little more expensive than a cheap under counter fixture.