Hi,
A quick search leads me to think these cost from $25 to $1,000. Some are portable and some I guess are very large and will not run on batteries.
Do you have one of your own? If you have one already and it is a plug in the wall device, unless it's designing the device that you want to do, maybe a simpler solution is some kind of timer plug.
If battery powered, I'd personally be reluctant to tamper with it to try and insert a timer of my own making onto/into the power on/off button, unless I understood the circuit enough, but it's a thought.