I recently purchased a Trek 820 "Antelope" mountain bike. I'm gonna be converting this into a heavy duty touring bike. I took a quill stem and drops off of a Raleigh I'm fixing up and put them on the Trek. The stem is 100mm length, 150mm height. I'm trying to figure out what size stem I need. Can someone let me know where I can find bigger quill stems? I have pictures here. I'll appreciate your comments and/or expertise.

What do you mean by "bigger"? Higher? Longer? Larger diameter quill?

I'm looking for one that is taller and has more reach. The Trek has a headtube angle that leans pretty far back. Maybe something around 225mm height, 130mm length.

From the photos, I'm guessing you're looking for a "road-style" angled stem, with an about 72-degree bend. For more height, Nitto Technomics are a common option, but I think they (and most quill stems I've seen, for that matter) top out at 120 mm.

The Nitto Technomic Deluxe has a 190mm quill and is cold forged. The cheaper Technomic is even longer, but it's gravity cast. The Deluxe has a much nicer finish and is only a few dollars more.

Oop, and to clarify, I meant 120 mm length. Technomics are famously tall...200+ mm of column!

https://www.peterwhitecycles.com/stems.asp

This page notes a few other differences between the Technomic and Technomic deluxe, in addition to what Dirtdrop mentioned.

Oh, and I understand there is such a thing as too tall a stem, so watch out for what's illustrated at the bottom of this page:

https://www.sheldonbrown.com/handsup.html