Are there prepaid cards for Boost Mobile?

I want to get a Boost Mobile phone with the option to buy a prepaid card, as I need it. I go to Mexico often, and Boost/ Nextel has coverage in the border cities. But, I don't want to change my current carrier (which doesn't have coverage there) to either of these. Therefore, I was wondering if Boost offers prepaid cards so I can buy them before I go. If anyone can help me with an answer, I would appreciate it. Thanks in advance!