S&W parts for the 4506:
Part # 203620000 $18.82 GRIP, STRAIGHT
Part # 203590000 $18.82 GRIP, CURVED
The listed prices are without tax & shipping, from the current list. Of course, that doesn't mean the price hasn't gone up and they haven't had time to update the website parts list, though.
No guarantee they still stock the curved one, either. It's not listed as an option for the 4566TSW and I haven't ever tried ordering one. They might still have some, since they will still make small runs of these older guns for agencies who are still using them. You'd have to call and ask.
Factory website link for parts lists (pick a format you like):
http://www.smith-wesson.com/webapp/wcs/stores/servlet/C...3016§ionId=10504