Have an old one around that has been cut, using a hacksaw...
Fit into place and measure to determine how much more or less is needed..
This is accurate....and easy
The source of a new belt
Graingers - a baffling selection and excellent service..
Beltsforless.com ( I think) (Google result) ...very informative and well organized.. from Belarus.... excellent presentation, well organized..
Sadly, Ace Hardware(as good as they are, otherwise) fell flat on their face...cannot compete....sadly.
How to measure :
Use an old belt, cut...measure this and use to determine how much more or less is required, I have found this to be accurate....fast and easy...