I finally ordered the Spey line! Mastery Skagit Spey Delux and Monocore Shooting Line both from Scientific Anglers. Dave recommended to put 10 wt line for my Redington 8/9 spey rod. I should get it by the end of this week.
The plan for the Christmas day is to go to Bonneville Hot Springs. It should be around two hours drive and ~400 meters hike (or snow shoe or cross country ski). It should be fun!