Just get a real amp designed for 120v, by the time you got a power supply to truly support a car amp you have surpassed the price of a decent amp designed for your purpose.
Look at plate amps on partsexpress.com, they are made for what you want to do.
^ Not Necessarily true, You could do what I do, I had a 600W computer power supply that now powers a 500w power acoustic amp for my sony xplod in my room.