Ideally we could re-use Ultimaker belt drive parts. There are some details on the specific parts here:
SDP-SI have a huge amount of applicable parts. In particular, these seem good:
The only issue is that they have a 5mm bore. It might be more applicable to Microrax to have a 3mm bore pulley.
These seem to fit the bill:
https://sdp-si.com/eStore/PartDetail.asp?Opener=Group&PartID=60080&GroupID=346, 17$ each, ~11$ for >50.
Those pulleys use GT2 belts, though I'm not sure what size would be applicable (belt length).
Ultimaker uses "240 mxl belt" and "80 mxl belt" (1/4in wide).