In Paris, you will have a rough time finding drinking fountains. I have only seen one. Ever. It's in the place Saint Michel on the north side, towards the river.
But unless you're planning to spend all of your time hanging out by Place Saint Michel, having access to water is helpful (without having to pay for a new bottle every time).
Save Money. Use fewer plastic bottles. Refill your own bottle at one of these lovely fountains.
They are all over the city, and they are (warning: information received from an unreliable source) supposedly the only fountains in the world where the water flows straight down rather than bubbling out of the top and then trickling down.
So, rather than experiencing what my father did (buying a 1 Euro bottle of water, only to find it was an unsealed bottle that the vendor had probably picked out of the trash and refilled), or going without water until your next meal, just make a pit stop every now again by the green fountains o' plenty.
I'll try to get a full-fountain shot on here so you've got a better idea of what you're looking for... but for the time being, this is all I've got.