Either salvage yard or you'll have to search the underside. I've dropped bolts before (PITA!) and they always seem to land on top of the tranny. I would be very hesitant to install a non OEM bolt there since you can open a whole can of worms (leaks, stripping where it goes into, etc...)
If you remove the battery & the front plastic cover (you may have already) and get a good flashlight & get in there & look.
Also, with a rubber mallet you might tap the underside of the bumper & air dam cover & see if you can hear the bolt rattle around. You might be able to pinpoint it better that way.
It's down there (unless you've driven it since) so don't give up!
