I had a short sale went into contract but it's taking forever to close because the seller bank still has not provided the consent letter.

Letter of Intent to lease or purchase2.

Send them a letter telling them you found their house to be apparently vacant.

If financing will be involved it's better to get that figured out and having a loan letter that includes how long it typically takes to close the loan and the maximum amount you qualified for before you make offers.  

(Make sure she doesn't do a deed-in-lieu, because some banks send out letters to homeowners who are in default, and if she does a deed-in-lieu, you've just lost a possible short sale.) 

Get a loan letter from your lender that states how long it will approximately take to close you and your maximum amount of a loan you qualified for.  

Is there any services that could send out letters for me?

User does not need any digits expect for the ones used to push the door bell (pun intended).Low tech option - put a standard lockbox on the door.

I have been sending out white letter mailers to multi families in my area.

I'm curious because I get sketchy letters in the mail all the time to RE-FI with a company I never heard of.