5A voltage regulators do exist, but are not very commen, I've been searching for them for a long time. National semiconductor makes them I believe, But I've seen only one company who sells them (And than only in large numbers or as sample's).
They come in 3.3V , 5V, and 12V with 5A. cost: about 1,50- Euro
P.S: If you have a 12 Volt battery, keep in mind that there always is a voltage drop across the regulator. With the best regulators (low-dropout) that voltage drop is about 1 Volt. So you have to have a Voltage of 13 Volt minimal to let the regulator function properly at all times.
It's true you can set multible regulators in parralel, but although they are of the same brand and type, they never are exactly the same. This makes one of the regulators do all the work, while the other one sits back and waits for the first one to run out of options, at 1A. At that point the second regulator will activate to accompany the first one. This way one of those regulators will "die" much faster than the other. This isn't really a problem but one of those regulators will probably be half way his melting point all the time, so you have to have good cooling.
The 1A regulators aren't really expensive anyway
By the way, I'm building a whole new computer DC-DC convertor for a school project. Sproggy like. As soon as it's working I'll try to put it on the net.