St. Croix reel spare spool

[bpoppa85]

Hey guys. Got a st. Croix rio Santo ready to fish fly rod package for Christmas. Came with a St. Croix STF55 reel loaded with floating line. Want to pick up a spare spool for it that I can put sinking line on. Can't find anything on the Internet. Anyone know have any ideas? New to the fly fishing game so I hope this isn't a terribly stupid and obvious question