Adjustable replacement strap for the MZ3 Physical Activity Belt. This band also suits and MZ1
 
*Note, the bands will say on them extra small for small and extra large for large. 
This is Myzone's sizing, but it's basically a small and a large.

It's important to note, that the medium/standard adjusts quite large and also quite small, so you really only need a small or large strap if you're sizing is a quite a bit outside of that range.