Reddit mentions of SUNLITE MTB Handlebar, 25.4mm Clamp / 27.5" Length / 3" Rise, Black

Stems are universal between brands, nothing to do with Trek or not.

Your bike uses the older quill standard for stems though. It either uses a 1" or 1 1/8" stem, from the photo it looks like 1 1/8", if the bike is from after about 1995 that's what it will be. These are fairly rare but there are some available on Amazon and eBay etc, there are also adapters available that let you run a modern threadless stem, though it might be hard to get high bars using one of those.

I would say though I would look at changing the handlebars more than the stem, to some riser bars, this was the thing to do back in the day when the bike was newer to change it to a less racey position than stock for greater comfort and control.

Riser bars add some sweep backwards and upwards as well as raising the bar height which helps more with getting a better ride position and reducing hand cramp, they also should be stucturally more safe/strong than using a really high stem or a stem extender, which often include usage warnings. These bars from Amazon are the sort of thing I'm talking about and would work - you would want the 25.4mm version to fit your current stem