There are tons on amazon. Check this one...
This is a very tricky question! Depending on the year make and model of the car it may be impossible to buy a shorter belt because of the path it travels.
If you're determined to try lay a flat 12 foot tape measure around the path of the belt without going around the air conditioner compressor.
Does your auto parts store list the belt for your year make model W/O without air? call and ask them or you can run a string around were the belt goes while allowing for the slack needed for the tensioner to take up and measure the string and bring the measurement with you to the store.
