How can I get a copy of my already signed 2017 amended tax form

if you haven't sent it in yet, there is only the TT facsimile of the return which you can print and then sign. if you sent it to the IRS then they have it but you can't get a signed copy only a transcript.   

An amended return had to be printed, signed by YOU in ink, and mailed in.  You can print it again.  If someone needs a signed copy, take out a pen and sign it.