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.  

The pickings at flea markets are slim and locating shots requires that you sift through mind-numbing amounts of "stuff". Look online for flea market listings for your area.

Shot glasses can also be found in antique shops and malls, but the likelihood of finding them very much depends on the area of the country you live in. A good strategy is to get to know all of the stores in your area and visit them frequently. Even in shot-parched areas of the country, shot glasses may appear and be snagged by a rival (or picker) within the space of a few days.  

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

Bottle shows and Advertising shows can also be fertile hunting grounds.  The Federation of Historical Bottle Collectors maintains a show calendar: check it for shows being held within reasonable driving distance and then plan on being there early. As in "very early"; early-buyer sessions are frequently held at 6 am.

Bottle shows are probably the best source of pre-pro shots in the wild, but be prepared to come away empty-handed.  Bottle dealers tend to be more aware of the interest in pre-pro shots than the average trader, but 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. 

In talking with antique dealers around the country, I typically hear that they find liquor advertising artifacts "difficult to move" and so they often pass when an opportunity to buy presents itself.  Obviously we need to be more vocal about our lust for pre-pro shots!