DuaneDegn:
The original motors you purchased didn't have a gearbox. It's very unusual for a DC motor to have enough torque to propel a robot without a gearbox.Pololu sells all sorts of nice little motors. They sell some without a gearbox but these are intended to be as replacements to be used with a gearbox. IMO, you shouldn't purchase motors to power a robot unless the motor includes a gearbox.
Oldsteve's strategy of purchasing a combination of wheel with gearmotor is a good one unless you know how to adapt wheels to various output shafts. It's harder than one would first expect to attach a wheel to a motor not specifically designed to fit on the motor.
The problem is, for my robots I really didn't think I needed a gear box because they are so small and they're only traveling on a 4 foot x 2 foot table with no load.
MarkT:
Those coreless motors are designed for small quadcopters and RC helicopter tail props - you'll find
props that fit the shaft easily - gear trains even, if you look for RC helicopter parts. Not my
first choice for robotics - micrometal motors are available in several voltages and many reduction
ratios and have metal gears, and are good value on eBay, for instance:
http://www.ebay.co.uk/itm/1Pcs-6V-100RPM-Micro-Torque-Gear-Box-Motor-Hot-Sale-/231466738662?hash=item35e47e23e6:g:r8wAAOSw1vlUyzNEBut first work out what speed and torque you actually need before choosing a motor, there's little need
for guesswork.
Really I thought that the pager motors I bought were going to have a well sized motor shaft to fit a wheel with a little work, but turns out they are really small. I wanted to find other motors in that size or a little bit bigger, and I really don't need a lot of torque, but enough speed so it doesn't crawl on my 4 x 2 foot table like a turtle, since my robots are only 5 x 7 centimeters...