You just need practice. I went from a non-towered airport to a towered airport inside the Baltimore-Washington ADIZ that requires LOTS of talking. I was very nervous at first, but got used to it pretty quickly. Actually, now I look forward to landing at Class C & D airports cause I get to talk to the controllers! I think it's fun...especially listening to the big boys.
Bottom line...I would definitely not spend any money on a training aid.