ebay is always the best place to check the price then do your serious shopping later.

do a search using "8T pinion gear" or "8teeth pinion gear" or something like that, there is a whole bunch of answers for your selection.
Be careful though, besides choosing the correct teeth count, the teeth pitch must be correct to whatever gearbox you are using.
Since RC cars are the dominate thing, most of the pinion gears are for the RC cars. If the gear's teeth pitch is correct, you will be fine.

good luck!