Monsters after 2002 share the same 305 mm shock that ST frame bikes used, so you could grab one from any 2002 to 2008 Monster or you can also use factory adjustable ST (ST2 for example) shocks. Consider yourself lucky, I spent the last week researching rear shocks and the older Monsters have way fewer options than your bike does. There are probably 10 really good choices on eBay right now for under a hundred bucks