Just fill in "username" and "domainname" fields, then save as vbs and run.
strComputer = "."
Set objWMIService = GetObject("winmgmts:\\" & strComputer & "\root\cimv2")
Set objAccount = objWMIService.Get _
("Win32_UserAccount.Name='username',Domain='domainname'")
Wscript.Echo objAccount.SID
Or use dsquery:
dsquery user domainroot -name username |dsget user -sid
If you want the results in a text just add "> path to file" like so:
dsquery user domainroot -name username |dsget user -sid >c:\usersid.txt