The motors I have been using so far have been taken from Cybot - a robot I built through collecting components as part of a subscription to Real Robots magazine. Unfortunately though, my extensive testing has left them a little bit worse for wear which is why DON-8r has been struggling to drive smoothly in a straight line.
I decided that I could do with some new motors so bought a selection, all of which were unfortunately no good compared to the geared up assembly from Cybot (despite trying a few hacks to get a bit more power).
The Real Robots magazine is now out of print but I thought I would chance it and contact the publishers to see if they had any back issues - which they did! A bit of email conversing and I received a big parcel in the post with the two issues I required and all the associated motors, gears and housings included along with them - perfect!
I meant to take a photo of the magazine as it would appear on the shelf but in my excitement I tore into the packaging and started building before I realised I should be documenting (as you can see above). So big thanks to Eaglemoss Publications for their friendly and efficient correspondence that has allowed me to get the motors I intend to use in my final model:
(as an afterthought I may also wire in a couple of potentiometers to allow the motor speeds to be accurately balanced and easily adjusted. If I have time…)