Please add you location to your user profile so we know where you are located. They are quite a few on craigslist. Shouldn't be hard to find..
Here's a few near you https://denver.craigslist.org/pts/d/buick-350-small-block-2-speed/6238339378.html https://denver.craigslist.org/pts/d/engine-350/6256118026.html https://cosprings.craigslist.org/pts/d/1971-buick-skylark-350ci/6224608797.html