I always use a backpack for MTB, but find it a drag on the road.
Many companies offer water bottle cage mounts that can strap/clamp to a downtube or seat post. I’ve had good luck with these.
I also used one of those double cage things that mounts under the saddle, but was not crazy about it.
Personally, I’d rather just carry some extra water than have to plan my ride around water stops.