it's a great question!!
My advice is to stick with the most common flowers - roses, lilies, or orchids, and in the most common colors (e.g. blue orchids are a challenge, but white or purple are feasible). Oh, and birds of paradise are plentiful there. They'll have carnations and such too. More unusual flowers like ranucullus or dahlias might be harder to find and thus even more expensive.
As Tiffany said, many DW brides go the real touch route and do fake vs. real. then there's no question about what you're getting, and you can take it home with you. That was my choice, and no one even realized they were fake. and now they sit on my mantle. (or heck, you can resell it if you aren't the sentimental type).
Okay everyone, one last off the wall question. Does anyone happen to know which flowers are in season in Mexico during October? I have been trying to google this for days now and I haven't found anything concrete or reliable.