Realistically, the actual concept of "fountain" is very rarely signed. Most often we simply ask "WATER, WHERE?" Or "I/me THIRSTY, WATER WHERE?" And the other person tells us where.
But if you do need to express the concept of a "drinking fountain" you sign WATER and then mime the use of a fountain.
DRINKING-FOUNTAIN:
Notes:
Years back most fountains had knobs. Then after the Americans With Disabilities Act was passed fountains started being built with surfaces that you press on (thus making the fountain easier to use if your hands can't grip or turn knobs).
See: THIRSTY
Notes: