Silver,
I'd send the whole rod to Sage since I think they'd need to match the ferrules for a perfect fit. Otherwise you risk breakage at the ferrule.

I notice on their web site, Sage states:
The Spare Tip Program
Sage is offering a spare tip program for your favorite Sage rods. Please contact our warranty department for pricing and availability. (Even though you want a butt section, not a tip...

You can call them at: (206) 842-6608 or toll free at (800) 533-3004

Or,

Send a letter to:
Sage Manufacturing
8500 Northeast Day Road
Bainbridge Island, WA 98110
USA