You can try a Radio Shack. but odds are your best place will be an electronics supply house (If in S.E. Michigan try RS-Electronics on Schoolcraft in Livonia).
This is the kind of store Joe's TV repair shops at.
NOTE: the switch you find may be rated for something other than 12 volts and if that is for a common T-8 Type florescent light fixture. I would want at least 5 amps contact rating. Switch might not be the same color.. Might be black.
But they should have one,, If you can not get the exact size, get a slighly larger one and file the opening.