Start free. Then go to cheap. Then, if absolutely necessary. shell out the big money. Escalation of force is the idea. Michael Pavlovich does do the best Zbrush tutorials, IMO. You can get the first series free I think on gumroad, and the following two are pretty cheap. I wouldn't start with his intro to zbrush though.…
In my personal opinion, Michael Pavlovich is the one stop shop. I personally learned ZBrush from his Eat3D tutorials/ video DVDs, but I know he also has a YouTube series that is probably very similar. You can check them out here: Part One Part Two Part Three
Either would be fine. If there isn't a tutor available, then online. If anything, you can just try learning the program online on your own, and check with a tutor or someone after, to see if maybe you're missing something or there's a workflow that you can work off of
Tip: Also ensure your're mainly referencing material authored by working professionals when self learning online, which to that end you're more than likely be exposed too current industry practices and workflows.