You can obtain an invitation letter for visitor visa purposes by asking someone who lives in the country you want to visit to write you such a letter. However, an invitation letter is not necessary to apply for a U.S. tourist visa, according to the U.S. Department of State.
If you apply for a tourist visa to visit Canada, you may be asked to provide an invitation letter from someone living there. This letter of invitation may need to be notarized in Canada before it is sent to you for inclusion in your application, according to Citizenship and Immigration Canada. The person writing the invitation letter also needs to include your complete name and address, date of birth, phone number and relationship to that person, and there needs to be information about the purpose of the trip, how long you plan to stay, where you plan on staying, how you plan to pay for the trip and when you intend to leave the country. In addition, the resident needs to include her own personal information, such as name, address, phone number, date of birth, occupation, status in Canada and details about her family composition.
When visiting Europe, an invitation letter can be one of the documents that you can show European Union border officials when they ask you for supporting documents in addition to your tourist visa, according to Your Europe.