Visa requirements for tourists may vary, depending on nationality. Many nationalities do not need to obtain visas if the reason for traveling to Mexico is tourism. This applies for citizens of the USA, Canada, European Union countries, as well as for Argentina, Australia, Brazil, Chile, Colombia, Costa Rica, Iceland, Israel, Japan, New Zealand, Norway, South Korea, Switzerland, Uruguay, and Venezuela, among others. People of these nationalities obtain a tourist permit, valid for the number of days they are going (or planning) to stay in Mexico (maximum 180 days). To get this tourist permit, you need to fill out a brief paper document when entering Mexico. This document will be stamped by the Mexican immigration, and you need to keep this until leaving Mexico. Usually, when arriving in Mexico by air, passengers receive this document on the plane, and it will be stamped when passing immigration. If you are arriving in Mexico at a land border, you have to look for the immigration yourself and ask for this paper document. You must also pay a fee of around USD 25 at the bank when entering Mexico by land (most border posts have an on-the-spot bank). If you enter Mexico by air, this fee is included in the airfare. Of course, you also need to present your passport when entering Mexico.
If clients do not have one of the above-mentioned nationalities, please check this link to see whether they need a visa or not: http://www.inm.gob.mx/index.php/page/Paises_No_Visa/en.html
If you require more information about visa for Mexico, contact a Mexican embassy or consulate, or you could contact your respective embassy in Mexico. Please note that SAT Mexico DMC cannot issue any type of visa.