Start by doing a web seach for "merchant account mexico".
You will find some results quickly that you can view and decide if the companies shown are worth pursuing. Once you narrow your search, do some homework before contacting these companies by using their name along with "scam" or "fees", etc. You'll be able to gather some good general information quickly.
Once you find companies that interest you contact them and ask for an email listing their fees, application costs, etc. If they cannot forward such an email cut them loose. Don't jump through a bunch of hoops and fill out long application forms just to discover their costs.
A good merchant account provider should be able to set you up with a fee structure similar to that of the U.S. If some tell you the account will be tagged as "high risk", cut them loose too. High risk means high fees. There are lots of money grabbers out there looking to make a quick buck by singing up anybody and everybody. You'll just pay them a bunch of money only to find them dissapear in a few months.
The world of credit card process is tricky business and takes some time to learn the rules of the game. The short version is that merchants are always getting the short end of the stick, but finance the entire structure of the system. The trick is to start out at the best vantage point then take your lumps as they come. So try to find a merchant account provider that has been around a long time and has good customer reviews.