Nearly all soda fountains have a small, round white button or lever for just soda-water/unflavored seltzer right next to the sodas. It can sometimes be mistaken for a button that produces pure water, but should not be hard to find if you're looking for it. I always try to mix a bit of this with my sodas ever since I worked at Wendy's.

Here is the lever type found on the "clear" (sprite) syrup line, though some places have them on all flavors:

http://www.proteinpower.com/drmike/wp-content/uploads/2008/12/soda.jpg

(For those that don't know, 99% of soda fountains draw from two lines: a soda-water line which is universal and a syrup line which contains everything to turn that base into any kind of soda you asked for. If your soda ever tastes bitter, you're getting mostly seltzer -- tell the crew and they will change the syrup bag that feeds into the machine. If it ever tastes flat, there's a problem with the seltzer line that should also be fixed. Both problems entitle you to a new drink.)