REI has Nuun, at least the Tempe store does. Look in the isle with all the sports nutrition products... it is in the farthest east set of shelves.
Most bike shops also carry Nuun, I know for a fact that all Landis Shops carry it (I go to the one on Southern and 101, NW Corner).
Besides being great for matches, NUUN is an exceptional product. It is in my probably "stolen alien technology" category because it works so damn well... I rely on other nutrition products for calories on long training days, but NUUN keeps me going when all I need is electrolytes here in the hellish season we call summer in Arizona. It has completely replaced other electrolyte replacement products I used to rely on. The tube is much more convenient than most other products and it mixes effortlessly on any temperature water (simply drop one in and forget it!). It has never been easier to bring electrolyte drink mix with you on a hike, run or ride before. (BTW, I have no affiliation with NUUN… but if a rep is reading this they should feel free to send me free stuff
)
Camelback does make a similar product that I have been using on/off since SHOT Show, but I have had such great success with NUUN that I keep going back to it when I know I have a long HOT training ride or hike ahead of me.
Edited to add: +1 to what Lukas posted about AirBourne!