First you should know that there are different types of anti-malaria medication - it is advisable to have a kit with a few of the different types.
Second, you can buy at least two of the types without a prescription from Farmacity or any other Farmacia. They are easy to find and most farmacias in BsAs do sell them.
Third, it is advisable to NOT take the pills before hand. The side-effects can be bad and not worth it. Malaria in the Amazon region is not as common as the media would have you believe. You are better off to have a cocktail of the appropriate pills handy, then if you do actually start getting malaria symptoms, start taking the pills in a heavy dose.
I can't remember what i paid for them the last time i bought them, but I think that one type of the medication was extremely cheap and the other was only moderately cheap. In any case the cost was a minor fraction of what it costs in North America or Europe.
Additionally, I encourage you to get some anti-dysentery medication as well. And of course, get a yellow fever inoculation.
You need to know the technical names of these drugs before you go to the farmacia so that you can ask for them by name. Do a little research on the internet and you will find what names the drugs are sold under.