Bottle holder for mountain bike
Get a clamp-on solution like the SKS Anywhere, Topeak VersaMount or Elite VIP. 

As Nate W pointed out in the comments, make sure it doesn’t interfere with your cables.



A quick and dirty solution with cable ties and duct tape can work temporarily as well.

There are also bottle cage mounts which clamp to the saddle’s rails, for example the TacX Saddle Clamp:


I think a disadvantage of this solution is that you can’t slide behind the saddle on descents. They are primarily intended for time trial bikes.