The barrel on my SP-10 is just shy of 25 inches, the receiver is 8 1/2 inches. That is a whole lotta gun out in front of you to get moving, but when you do, it don't stop.
I would not want a longer barrel, as this one works just fine.
Remington should still be selling barrels, as they do have replacements and parts for the SP-10.
If you want another barrel get it from the factory. This way you know the chokes are as they should be, not like on many guns that they did produce.