They occasionally show up on CL. I checked the local Des Moines and other Iowa CL ads every day for 18 months before I found one a year and a half ago. An aftermarket flat fiberglass lid showed up about the same time. I ended up buying both but still haven't used the lid. The topper (ARE) was only 5-6 miles from me, but I had to drive 125 miles each way for the lid. I got the topper from a young guy who bought the ST from his uncle and didn't want the topper. They knew both sides of the market - very few for sale, and very few buyers. I paid $600 for it. There are plenty of Sport Tracs around here, but I've only seen about 5 or 6 STs with toppers in this area, and I'm not sure how many are actually local. There's a black '02 on a small used car lot here with a black Leer topper. I've been tempted to see if they'd sell me the topper to put on my black '01, but I've pretty much lost the desire to have a topper on both trucks, especially since it already has a raised fiberglass lid that meets the need.
Be patient and diligent, and expand your search as far as you're willing to travel. And don't hold out for one that matches your truck. Another member got one painted for $125 recently.