The 29er mountain bike movement started really taking off around 2000. There were a few experiments by a couple small and large mfgrs in the 90s but commercially it didn't take off 'til the 2000s. The Wikipedia page for 29ers has a nice history of the format that is well-sourced.

So you won't find anything actually vintage. In terms of steel 29ers I would look at a Surly Krampus, which is a 29+ bike, meaning it has 3" tires, which is in between the traditional 2" width and the 4" you would see on a typical fatbike. The bike is rigid but you get a ton of float from the big tires, but they're not so big as to be slow like a fatbike. It's a very fast bike and rolls well over relatively technical terrain. They just updated the model this year and it is very nice, especially for the price. And you don't have to deal with the weight or maintenance of a suspension fork. Good option if you are not planning to do very technical/rocky trails.