Go to TwitInfo (Account Info for a Single Account)

Twitter ID/Screen Name Converter 

Type or paste a list of any combination of Twitter IDs and Twitter user names in the first box. The list should be plain text and the items on your list must each be on a separate line or the items must be separated with commas. Screen names can be with or without the "@" sign.  Hit "Go" to generate your results.
                  
  Paste:           All Results:                                                        Identifier List:                       User Link:                                   
              
Note:
This converter does not use the Twitter API so there are no limits to the number of items you can convert but the processing is slower.  Before you
paste a huge list into the box, start with a small list to get an idea of how long it will take.

Warning:
If your list contains illegal characters (anything other than commas and the letters and numbers normally found in a Twitter user name ), the server will warn you of a script error. Remove the illegal characters from your list, reload the page and try again!