Just buy a decent Hifi - dont forget that a 2000 watt amp is probably rated for about a quarter of that ie 500w. It probably will put out 2000 watts for a few seconds before it blows/ overheats / drains all the street lights by the time you have four / 5 speakers running off it it is putting out about 80-100 watts per channel which is what a good surround sound reciever will give.
you just want a 2k transformer. not a computer psu... or a battery charger and a battery. connect the charger to the battery and then the amp. if you dont use a battery your likely to blow the amp as the battery works as a kind of regulator.
Before you buy anything check your home fuses/breakers, figure 1amp = 125 watts, a typical fuse/breaker for any room except utility/kitchen, should be 15Amps, which would give you 1875Watts, normally you don't want to exceed 80% of the fuses capability. For this case, 1500 watts would be your max on a 15A fuse/breaker.
Amps x Volts = Watts
Remember this is EVERYTHING on that fuse/breaker, which is normally two rooms per fuse/breaker, so if you max out on one electrical socket (which I don't recommend anything over 1300watts on an individual plug), nothing else can be using electricity or you'll blow the fuse.
Too many Amps on a fuse/breaker can cause significant damage to your homes electrical system, depending on how old your home is, wire guage etc...
Also, keep in mind that a lot of car stereo shops will oversell wattage on cars... based on A x V = W, as stated above, a 2000watt Amp would require a 165A Alternator in a car. Alternators of this size are typically found on large diesel trucks, whereas 120A's on Pickups, and typically 60-80A on cars (stock). Cars/Trucks usually are 12Volts, to mimimalize the compexity of vehichle electrical systems.
You would probably be fine pushing a 1000watt PSU into the Amp, however check the 12V rails for maximum wattage, even if you got 700Watts, that would be about the maximum you could get from a car alternator if you never upgraded it. (Meaning little to no change from what was in your car, assuming no upgrades).
Also on fuses/breakers, typical homes with 15Amps have 14 guage wiring, and 20Amps 12 guage, do not exceed the maximum specification of Amps on the wire or you'll probably start a fire. Scratch that: You will start a fire if you exceed the amps/volts on a what a wire is specified at.