I doubt that you could purchase something ready-made that does this.
If you have some experience with electricity you could build something like this:
This does exactly what you want. The relay and the push-button must be rated for the tension of your power supply and the contacts of the relay and the push-button must be able to withstand the current consumption of your computer and Spooky equipment connected to the side indicated by Power supply out.
It works very simple:
On connecting the power supply to the input there is no power on the output. After pushing the button the relay energizes. The power on the input is switched trough to the output. As long as there is power on the input the relay remains energized. As soon as there is no longer power on the input the relay is no longer energized and the connection between input and output is broken. The power remains off the output till there is again power on the input and the push-button is pushed.
For more details, please check the link: