Authorized Dealers
The best way is to negotiate with an authorized dealer. As collectibles are their business, authorized dealers will have the best stock, many hard-to-find items and will have their merchandise in the best possible condition. Authorized dealers advertise in sports card price guides (like Beckett and Tuff-Stuff), attend card and memorabilia shows and often have their own website detailing their stock.
Card Shows
Card shows are a great place to meet amateur and professional card collectors. Authorized dealers will have booths set up displaying their merchandise (look for the vintage baseball cards under glass tables). Attendees will frequently bring old cards to sell or trade. Events like these are great ways to attain rare finds and to network with others who share the same interests and who have access to vintage material.
Antique Shows
Baseball cards have been around since the late 1800s and have historical value. As such, they are of interest to antique dealers and buyers, specifically ones dealing with popular culture items, and can be found in some antique shows. Buyers will likely have better results at larger antique events, but occasionally a set of rare vintage cards can be found at smaller locally organized shows.
Garage Sales
Fishing around at garage and yard sales takes a lot patience and a lot of luck. Most, if not all, old baseball cards found at these informal sales will be of little retail value because of the poor condition of the items and the fact that most of them will be "commons" (cards depicting average baseball players). However, if one looks long enough, he may eventually stumble across some good quality vintage cards at very low prices.
Hobby Shops
Hobby shops, specifically ones dealing with sports memorabilia, are local hubs for those looking to buy, sell or trade baseball cards. Shop owners may not be authorized dealers, though, and if they hold vintage cards of potential value, it would be a good idea to inquire about their authenticity. A reliable store will be able to put card-seekers in contact with vintage card dealers or possibly even acquire cards on the patron's behalf.
eBay
Dealing via eBay involves the risks of doing business over the Internet. Be sure to pay attention to a seller's approval rating and testimonials. Reliable authorized dealers are putting their merchandise up for sale on eBay more frequently. Under the auction format, buyers can purchase individual cards and entire sets at very good prices (or, conversely, get into a bidding war with another prospective buyer and pay too much).