Breeding's basically the only way I can think of for some species, like Riolu and Lucario.

[...Breeding. Hrngh, pardon Riku's Lucario if she grumbles something under her breath that may or may not sound like "tell me about it" in plain English.]

But species that can't breed without Ditto are pretty much out of luck. That is, unless they end up with a trainer with one of those TMs for a little while.