It's kinda hard to remember the specifics (since I'm thinking back 20 years now) but I remember seeing the mail-in order preview for Tahu and Vakama in a 2001 Lego magazine, before the sets actually hit U.S. shelves (After a quick search, managed to find the specific issue too)
I don't think Tahu was my first one, though. Since I have a twin brother, it was actually pretty easy to collect a whole Toa set (Thanks to birthday gifts, we were able to get a lot of them at the same time) I seem to remember getting Lewa, Gali, and Kopaka first, and Tahu, Onua and Pohatu later. I know that order specifically since we could build those funny-looking combiner sets.
Oh, and we got duplicate Matoro sets from McDonald's. Yeah, imagine that.