I searched for the words "honda cm400".
Right now, there are at least five regulator/rectifiers up for auction, none over $10.
This is the excellent quality Shindengen unit successfully used by me and many others to replace the pathetic Suzuki part.
STF for more complete instructions, but here's how the wiring goes:
Three yellow => stator wires in any order
Green => ground
Black => SWITCHED (live only when the key is on) positive +12V. Splice in to a tail light wire or somesuch.
Red => Battery positive +12V (behind a fuse, please). You can use the existing wire for this.
My bike was sorta OK with the stock part (13.5V max, visible flickering), but the new R/R rocks - a solid, steady 14.7V anywhere above idle. It also runs much cooler.