If your looking for Grass Carp in the St. Louis Region you can find them in any of the creeks and backwaters of the larger rivers including the Missouri, Mississippi, Meramec and Big Rivers. As far as lake fishing,  I know Busch Wildlife lakes 37 and 38 have them.  To find them go to the back of the lakes, you will need to rent a boat.  They can be quite difficult and spooky, I recommend lake 38 as the water is more off colored and the fish will not spook as easily.  There are also fewer weeds which will make it easier to land fish.