You need to find a dyno shop. They'll run your car on a chassis dyno - basically run it on rollers. By measuring the rate at which your car can accelerate the rollers, the HP & torque available at the drive wheels is calculated.

Dynojet makes the most commonly used equipment. Point your web browser to http://www.dynojet.com Select automotive dynomometers, then the "support channel" pull down menu. You'll find a locator for the nearest shop(s) there.

You'll have to call for price, but two quick runs and a printout graph run $50-$75 in my experience.