Drive Gear flew off and I can't find it... replacement recommendation?

It matters a bit since if the one you buy has a smaller or larger diameter inner hole it will not mate onto the drive shaft (but most NEMA 17 steppers for printers have common shaft sizes).

It also matters if the gear diameter is different since your steps/mm for the extruder will change. The outer diameter on the one you linked is 12mm and the one on the C2/R2 is closer to 9mm (9.1 is what my micrometer says with a quick check). I do not know how may teeth are on it.

 

This looks closer: https://smile.amazon.com/TOOTO-Direct-Extruder-Filament-Printer/dp/B01DEZZIHW

But I would get a replacement from Robo even if you try one of these.

 

Almost all direct drive extruders like this design use a Reprap style hobbed drive gear. Just a quick summary, they are usually sold by series labeled as MK:

For a chart, with pictures of the most common types look here: http://reprap.org/wiki/Drive-gear. The links to sellers on the wiki page are old and pricing is better on Amazon (N.A.) and China.

The one Mark pointed to appears to be an equivalent to the one used on the R2/C2 and as he mentioned most NEMA 17 steppers have a 5 mm drive shaft (with geared steppers being the exception).