#Access from behind If you have access to the back side of the electrical boxes, you can seal them using a moldable putty in sheet form. 3M™ manufacturers 3M™ Fire Barrier Moldable Putty Pads, which are intended to be used as a firestop but will also block air flow.
This is probably a good thing to do anyway, since garage walls/ceilings should be a firestop anyway.
#No access from behind If you don't have access to the back of the boxes, you'll have to seal them from the inside. To do this, you'll have to use a couple different products. Seal the openings where the cables enter the box using Duct Seal, making sure to get a good seal all the way around the cables. Duct Seal is a permanently soft, non-toxic compound, that can be molded to seal around cable penetrations.
Then install foam gaskets under the cover plates.
