It's basically done just how us homebrewers can do it at home, only a little more sophisticated.
Basically... set a petri dish out that has a gel medium on it. Wait for yeast colonies to form. Scrape from petri dish and add to small starter. Package starter. Send to LHBS.
Keep in mind that most of the yeast strains you see in LHBS's have been produced for decades. These strains are probably not "caught" using a petri dish, but reproduced from current stock.