In Taim's case it's likely he may have been taught by a Forsaken, since he seems to be a Darkfriend. Possibly Demandred, since he echoed Dem's order to "let the lord of chaos rule".

It's also possible he was found by sisters of the Black Ajah, or that the rumors Red sisters encouraged the False Dragons to earn prestige in capturing them, were indeed true. Though this seems less likely since saidin/saidar are too different for a sister to have taught him much. I'm gonna go w/Demandred, he was probably the one to order Black sisers to free Taim so Taim could be a plant in the Black Tower.

As others have said, the male channelers who declare themselves the Dragon are all very high in strength, so they may have just taught themselves. They'd have to be strong mentally in the first place just to handle saidin at all so they're probably just more suited to channeling without burning out/dying. I also agree with the idea that they may have been in a position to have learned the flame/void trick Tam taught Rand which is an initial step to channel. Could be that's how they learned they could in the first place.

Just finished reading Patricia Briggs' newest Mercy Thompson novel, River Marked. I've really liked the other ones as far as light reading goes, this one felt shorter and more rushed though.

Now starting on some of the Valdemar books by one of my favorite authors, Mercedes Lackey. After that I may read more of her books, go find and re-read my copy of New Spring, or possibly go back to a post-apocalyptic series I began awhile back, The Vampire Earth by E.E. Knight. All very good, if all over the place in terms of style/theme!