Cannot find efficient current/voltage regulator to power LEDs

I have a 12V 4-pin common-anode RGB PWM'd signal being used to power some LED strips, and I would like to use it to power the VL-H01RGB00302 Vollong 3W RGB High Power LED – which is basically a set of 3 LEDs:

forward current and voltage:
  Red:          400mA, 2.5V
  Green, Blue:  350mA, 3.4V

Unfortunately heat dissipation would be a major issue if I merely used resistors: perhaps 10W wasted as heat ((12V-2.5V)400mA + 2(12V-3.4V)*350mA), which I am loathe to do even with a heatsink. I tried it on some high-power resistors and it gets really hot (mirage illusion hot).

I consequently have been looking for a few months for a very small driver/regulator circuit (perhaps at most a cubic inch) that is very efficient and therefore produces almost no heat, and can be powered from the 12V without requiring its own power supply. I imagined that I might be able to use a switching (or other highly efficient, e.g. 90-95%+ efficiency, non-linear) voltage or current regulator, and just pretend the PWM signal is the power supply. (I would assume it would work if it had a fast power-on/off-time and no built-in anti-ripple capacitors or other mechanisms.)

I would prefer a component with long pins I can just carefully bend and screw-mount onto a binding post, and something with minimal wiring. Though the holy grail of this would be a current regulator, I've also considered a 4.5V voltage regulator… but that still forces one to dissipate something like 2W across the resistors.

The issue is that I have not been able to find such a device anywhere. Almost all regulators specsheets I see hide their efficiency (or do not provide the information necessary to determine it for various operating ranges). The efficient ones I've found specsheets of I cannot find where to order from (goggling for the part name reveals nowhere I can buy them). I've also looked for efficient DC-DC converters and haven't been able to find any (except one, which I bought which unfortunately would not work with PWM).

So my question would be:
What is the easiest way to do this? Are these the components I am looking for? Where does one get such components?

(I could make a complicated circuit perhaps that uses the PWM as input to modulate a brand new 3.4V signal, but have minimal experience with complicated designs, and due to wiring issues (have to hang this), I'd rather just keep the 12V 4-pin common-anode RGB PWM as the only electrical input to the system. I would consider all simple solutions though.)