where can i get travel papers for a single mom to travel with child, without father?

If there is a father you must have I believe a letter allowing you to travel to Mexico with the child whether you are living together or not.

Ia there a form letter?

If the child was born in the US, you should be able to get a passport for him/her or add to your passport assuming you're a US citizen (I think) but I'm NOT a lawyer. You might post this question on the Caribbean board here which covers Mexico. Speaking to a lawyer is probably the best idea.

You don't need to contact a lawyer. To Mexico, you don't need a passport just US Birth Certificate and photo Id. (Not sure about this for minor's.)

The restriction on a child entering or leaving Mexico with one parent is THEIR restriction, not a US restriction. I'd suggest you contact the nearest Mexican consulate as to the apporpriate documentation.

You need a notarized letter from the father,complete with raised seal.Mexico is big on raised seals.If you are divorced and have custody you will need a copy of the divorce decree.If the father is deceased you will need a notarized copy of the death certificate,again complete with raised seal.Many notaries use the non-raised seal type of stamp.That won't work for Mexico.

You can also check with the airline you booked. My daughter travelled to Mexico with a school group, and I believe both parents had to sign a form letter.