I have phone numbers and e-mail addresses but it's best to write. You might want to use post cards. I recommend the plain, post-paid ones available at any post office. On the reverse - the message field - write something like, "I'm seeking information on my uncle, James E. Ferguson. He was a medic and was from Texas. Was he a member of the division (or regiment, as applicable)?" Keep it simple. Most of these guys love to assist in situations like this and will send you everything they have without your having to furnish lot of details.