I have a BM new style tele-stock on my preban Colt and it works very well. You can get the actual tele-stock Colt uses on their LEO and M4 rifles from several dealers, R-Guns comes to mind, for around $125 to $150. Look on the FS board and you can get used ones a little less.
Just about any tele-stock will work, IMO I would stay away from gun show specials as they are not near as good quality as a Colt, BM, RRA product. I would also stay away from the Olympic Arms stocks.