Finding pre-pro shot glasses in the wild requires patience and diligence.  There is no single best source, but they're found most frequently at flea markets, antique stores and bottle shows.  

www.fleamarketguide.com provides a partial listing of flea markets in your area.  Shot glasses can also be found in antique shops and malls, but, depending on where you live, pickings may be slim.  A good strategy is to get to know all of the stores in your area and visit them frequently for shot glasses may appear and then be snagged by a rival within the space of a few days.  

Serious collectors may want to get a “business” card made up and distribute them at antique stores so that they're notified when shots come up for sale. 

Bottle shows and Advertising shows can also be fertile hunting grounds.  Antique Bottle and Glass Magazine publishes a calendar of bottle shows  around the country, but be prepared to come away from bottle show empty-handed.  While bottle dealers tend to be more aware of the interest in pre-pro shots that the average antique trader, they're finding them increasingly difficult to obtain and prices that they have to pay for them in recent years  makes them to reluctant to buy them. 

My own hunting grounds include Pennsylvania, New Jersey, Delaware and Maryland.  In talking with antique dealers, I typically hear that they find liquor advertising artifacts "difficult to move" and so they usually pass when an opportunity to buy presents itself.  Obviously we need to be more vocal about our lust for pre-pro shots!