Assuming that you have a 7.1 channel analog output to connect the amplified system to I would suggest you check out amp modules at Parts-Express.com
You can put together the right combination of boards to get the right amount of power. The sub will need more power than the sats. You will need a power supply too.
Don't forget that the amps will need to be kept cool so building them into the sub might require some external heat sinks or a fan.