It's really simple: buy the LED
, attach to a 3.5V source (a pair of AA's will do close enough at 3.0V).
The thing seems to have current limiting circuitry built into its IC. I've hooked it up to 9V directly for some time. It remained healthy. But I'd be reluctant to close it up in a model that way.
I wire 9V, so I put a 3.9V zener diode
across the LED in reverse bias (negative to positive). The zener acts as a "safety valve" shunting excess voltage around the LED. In addition a 100ohm resister before the LED/zener combiniation. This gives the LED a constant 3.9V as it cycles through the colors and current varies a lot.
Here's a pic of the "reactor's" backside. I've painted over the butt of the LED. The positive lead is on top. Note the negative side of the zener diode is towards the positive LED, and the 100ohm resistor is on the positive side. Connect to 9V source (battery) and watch it go!