Ouch that stinks, sorry to hear about your rod tip. Don't feel too bad, this does happen--- casting weighted flies (like clousers) that can fracture a blank if they whack it, overhead ceiling fans, car doors, size 12 feet, trying to free snagged flies from tree limbs etc will gobble up more rods every year than breaking them on monster fish.
You'd really want to get a tip for that specific brand/model/weight blank as opposed to a tip from another fly rod that might happen to fit (and I wouldn't even bother with attempting to repair the broken tip portion).
So you'll definitely want to contact the fly shop you bought it from if purchased recently, or you can go to St Croix to see about repair or replacement-- the Rio Santo rods have a 5 year warranty that might apply depending on the circumstances (bought new, registered, etc). But they have sort of a tiered warranty/replacement program that also lets you get a repair replacement for a nominal cost (50 bucks) or a replace/upgrade (old St Croix rod for a new St Croix rod). Here's a brief run down of their policies:
Silver - for "defective" rods under warranty 20 bucks
Gold - repair (new tip in your case) or replacement for 50 bucks
Gold Star Plus Upgrade - If worse comes to worse St Croix does have an upgrade program, which lets you purchase a brand new St Croix with a bit of a discount. It works like this (Cost of New Rod + 50 bucks) - Cost of broken rod
Here's a link to their site: St. Croix Rods - Service and Warranty
If you bought it new from a local shop they might be able to replace it for you free or for a nominal charge, or help with contacting St Croix on your behalf.
Good luck, let us know how this pans out.