McMaster.com
The part numbers are:
X and Y axis: 7959K24 (sold by the foot - you'll need 8ft)
Z Axis: 6484K512 (the wrong belt is listed in the BOM - it lists the 37" ones, which are too short - this is the 39" belt).
Do keep in mind you'll need to split the belts - these are roughly 6.5mm, and you need 5mm wide ones for the Mendel.