Buy at home. Take them with you. You won't save substantially buying enroute, and the likelihood of ending up with out of date, mis-stored and/or counterfeit medication would be greater.
That said, go to Lonelyplanet.com/thorntree, find the "Health" branch of the forum and look for posts about malaria. There are hundreds of threads. In particular, pay attention to the most reliable poster there, Nutraxfornerves. She's reliable, balanced, knows her stuff and doesn't fall prey to silly sensationalism.
Note that chloroquine is probably not a good choice for parts of Latin America. Make an informed choice.
Hope that helps.