Do you have the old belt? If it's not OEM, it may have a # on it.
Otherwise, if it's a standard width/depth and the length is the issue, any decent auto parts store may be able to hook you up. When I last needed a belt for a mower, I went to Advanced Auto and the guy measured mine and said "Sorry, we don't have any at 150-inches but have belts in 1-inch increments up to about 110".