Cheap t-shirts (Las Vegas type, not the big fight specifically) can be found in the Bonanza Gift Store (Large souvenir shop on LV Blvd and Sahara), on Fremont Street (Duece or SDX bus will take you there from the strip), or in the Hawaiian Marketplace (between Planet Hollywood and MGM on LV Blvd). They are pretty much always the same price in these locations ($4 dollars and up). You may also be able to find some inexpensive stuff in CVS or Walgreens on the strip. The only place that you will likely find Mayweather / Pacquiao t-shirts and other things with that name specifically is at the event itself. Expect to pay big $$$ for those things.