Henning Group makes aftermarket magazine springs for several gun models.  They are high quality and always fit factory followers.
 
These are recommended for 140mm magazines (20 round) with an extension.
 
If the spring tension is too high for your particular setup simply cut coils until the desired tension is achieved.
 
 
Check out our magazine extension database for 2011 / DS 1911 extensions that will go perfectly with these springs!