I've been looking for years... finally found something.
Search 'collet extension'.
http://www.ebay.com/itm/271742108328
DZ BRAND 5/8" ER11-150 COLLET EXTENSION
(maybe it doesn't hold die grinder collets....)
If so: http://www.ebay.com/itm/261578827020
New Collet Chucks Precision ER11...