The FMT (official Mexican Spanish abbreviation for what we refer to as a "tourist card") is a permit required of visitors staying longer than 72 hours (including the border zone) or traveling outside the US/Mexico border zone (regardless of length of stay). They are granted for up to a maximum of 180 days by Mexican Immigration.

There is most assuredly a fee for the FMT - it is approximately US$22 (depending on the current exchange rate) but the fee is included in the multitude of taxes and fees that your pay for international airline or cruise ship tickets. If you cross the land border you will need to pay the fee to obtain the FMT from Mexican Immigration.

You can wait until arriving at Tijuana's airport and get your FMT from the airline as previously posted. What is not clear to me is whether or not you've already paid for the FMT or not given that your flight is domestic and not international. So don't be too surprised if you have to pay at the airport.

Whatever you do don't lose the FMT and remember to turn it back in to the airline when leaving Mexico. If you lose the FMT you will need to replace it or face a possible fine.