I believe the term you're looking for is "Spinner".
I've bought mine in past from
Value Hobby, but nearly every decent hobby shop will have at least a few sizes of them, if not a few colors as well.
The size is spec'ed on the width at the base, so which size you need will change a bit with he airframe. IIRC, I used a 2.5" spinner on my FT spitfire, and the front of the cowling on most of the larger FT planes will be close to this.