I think I have the same motor. You don't measure between the flats, you measure the outer circumference of the shaft. Mine is a 5mm shaft. You probably want the metric plastic spur gears.
I'm in Australia, so shipping is usually about 10x the cost of the actual product.
Good Luck.
Rob.
Well, drat.
On a slightly separate issue, my project doesn't look like battery operation is a viable option.
Two of these steppers on 8 step motion on an Arduino Nano --- runs at ~500mA.
500mA is a bit much for any kind of lifespan out of AA batteries.
sigh
Backup option of plug in power it is.
t'll run on batteries, just not for more than 4 hours or so.