First of all you must realize that both these men are extremely busy and are deluged with requests from every quarter, every day.
If you really want to say something to them a letter, yes a good old fashioned paper letter in an envelope, is your best bet. You will have to dig around to find a postal address, or try by sending the letter to their name care of NASA. In any case, unless your letter is interesting to them you may get nothing back, or a form letter, or if you really had something unique to say and said it well then perhaps you might get a real response.
The chance of getting a reply from Buzz Aldrin is slightly better than from Neil Armstrong as he has been out and about while Neil is a very private person and has refused numerous interviews and guest appearances with reknowned and famous persons for over twenty years.
So, its not impossible, but I doubt anyone here is going to give you a shortcut to the ear of these pioneers of spaceflight.
L