Where should go to get a invitation larter for invite some one from onther countries?

You simply write a letter stating your name address, and that you are inviting a certain person to be your guest in the U.S. Give the dates the person will be spending with you. State that the person will be your guest and you will provide a place to stay while he/she is in the U.S. State that the purpose of the visit will be to spend time with you and tourism . It does not need to be long...just tell where that person will be staying and with whom. The letter is not required, but the person requesing the visa may offer this letter when he/she goes for their interview. The sites below have sample letters.