Where could i write some letters?

Where could i write some letters?
for a project im doing, i choose to write a letter to a organization. i was suggested to write to AI, but i have no idea what to write to them. i was also thinking about writing a thank you letter, but i do not know where to write it to. i was wondering what are some good places to write to (ex peta) and what i could include in that letter/what to write