A lot of chainguides come with adapters these days, so you might just buy one but check before if it's included. I wouldn't spend that much money on an adapter alone, when you can add a few pounds more and get a full guide that is used but in good shape.
http://www.chainreactioncycles.com/Mode ... elID=79648
1990 GT Karakoram 20" Purple Haze (urban project); 1993 GT Karakoram 20" Ferrari Red (classy beater); 1993 GT Karakoram 14.5" Black Light (almost original); 1994 Breezer Jet Stream 18" (unlikely story); 1995 Winora full suspension (a fine riding mistery); 1997 Kona A'Ha (anniversary edition); 1997 GT Arrowhead 18" (poisoned gift); 1998 Stevens 6.1.2 (true warrior); 1998 GT Moto cruiser (just google it!); 1999 Santa Cruz Chameleon (the heartbreaker revived); 1999 GT Lobo 1000 (retired with works Super T); 2000 GT Lobo (turns original); 2001 Klein Attitude Comp 19" (ultralight project); 2001 Cannondale Jekyll 1000 SL (better that a lot of "fresh" bikes); 2003 Intense Tazer (close to factory team spec); 2003 Intense M1 (old dream on hold); 2007 Crupi Pro XXL (pumptrack teacher)