Best design choice: linear regulator or switch converter
Your ripple requirement isn't that hard – what's more problematic is that your output voltage * output current = 300 W!

That's quite a lot. You would not want to burn a couple of volts at 20A over a linear regulator (which, by the way, makes no sense – that will convert the energy that you don't put inot your thermoelectric module to thermal energy, which is kind of what you wanted to regulate in the first place....).

Linear regulators simply work by having an "adjusting" internal resistance that simply drops the voltage difference that's between your in- and output and converts that to heat. So, if the in-output difference is just 2V, at 20A, your linear regulator would dissipate 2 V * 20 A = 40 W of power. That's a terrible thing to cool.


  If the linear regulator is a bad idea, is there any off-the-shelf step down switching regulator which does not require any external components (no coils or caps) more than a few resistors? 


Terminology: When talking about regulators, it's not perfectly sure whether you're only referring to the thing that regulates the currents flowing, or mean the complete system including all the necessary energy-storage components. Usually, we'd use the former meaning. For the other thing (controller + switch + power storage (coil)), we'd say supply, or at least module.

You can of course buy readily made power supplies. Every laptop has one, and they even exist for the currents you need. Getting one that is adjustable might be a little harder, but you might want to think about just using PWM on the output to reduce the average power going into your module. Of course, that'll technically absolutely break the "5% ripple" requirement (PWM is actually 100% ripple, if you want to consider it that way), but I'm not sure where that requirement came from in the first place. Maybe you'd want to also specify the acceptable/unacceptable frequencies for deviations from the intended current/voltage point, and explain why you'd need so strict regulatory limits for something as slow as a thermal element. **Update:* nope, not PWM then, according to your comment :)

You can also buy adjustable 300W supplies – but these tend to be a little more costly.

Regarding modules: The module we're talking about will most probably be sold as "open|closed frame power supply".