2-3gpm is about right for garden hoses, so that is what you will probably find at big box hardware stores.

Do you live at the pond?

Even 25gpm is not an outrageous rate to fill a pond.

Can't you just let it run with the valve full open overnight? If not, maybe pinch the valve closed a bit.

If you must have a mechanical timer, you may need to go to a rural irrigation store. (Like the guys that carry equipment for center pivot irrigation.) Most industrial things are controlled on the electrical side, rather than mechanical. But they might have something?

Finally, do you have "time of day" metering on your electricity? The electric charges are usually the highest cost item on a deep water well. If you have to run a lot during a drought, perhaps you can work a deal with your electrical provider for a reduced rate, if you only run the well continuously at night?

Good luck on your pond project!