Are you pushing things into the motor, or pulling things out, or just worrying about the sideways (radial, normal to the axle) load?
For radial load, I would guesstimate by looking up 6mm inner diameter ball bearings, finding the cheapest version, and looking at their rated capacity, then divide by half. Or perhaps open up one of the motors, look at the bearing, and see if you can figure out which specific bearing it is?
If you will put substantial load on the axle, I would suggest putting another ball bearing on the other end of the load -- perhaps even a bearing on either end, to de-couple the motor. That way, you can define the load capacity yourself by just choosing the appropriate bearings.
Here's one such bearing:
Rated at 440 N static load.
There are lower-cost bearings on that site, too, but they don't display a rating. And I don't know how good a comparison this is, anyway -- if our Pololu friends could give the real specs, that would be great!