This is a great link, as it has all the links for the herbaria in Mexico, also in southern USA, such as Texas etc. If you then want to access all their databases, you go to the bottom of the page, click on "Access to the information", say you accept and then start searching. Maximise "Vascular Plants and Germplasm", select all herbaria and you will get outputs of all specimens databased for these herbaria. You can output in 2 different forms, comma delimited is great for copying and pasting into wordpad and then adding to excel and then being to convert the degrees to decimals and adding to ArcMap. Enjoy!