How can I seal electric outlets to prevent garage fumes from seeping through?
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.