I thought there was a spigot right off the road on the right as you're heading up, between the road and the ranger kiosk. I can't tell from Google Maps, so I may be crazy.

There should still be actual restrooms with a drinking fountain off to the right a few hundred yards, too. Turn right at the first driveway after Quimby.

I never stop there because I'm a bit of a camel and save all of the water for the big climb after Grant. But this is what I remember from hiking trips with my wife.