There's a small 3D-printing niche of PET #1 bottle cutting specifically for filament making. For whatever reason, they mostly seem to organize on Facebook.[1] A lot of these builds can be made out of scrap components.
The Recreator3D[2] somewhat automates the bottle cutting, feeding, and filament making. Older versions repurpose a specific 3D printer's hardware, but the MK6[3] uses largely off-the-shelf parts.
Igor Tylman sells kit and fully-assembled PETmachines that cut, feed, and pullstrude filament.[4]
(worth noting that these pullstruders are all separate from the methods of shredding or grinding plastics into granules/pellets and using an extruder, i.e. https://www.youtube.com/watch?v=nQ0UyWKafAw, whose videos on the subject are good overviews of the process but tend to annoy me because they use pretty hobby-accessible tools for everything but the critical grinding part, for which he wheels out a sponsor's $10,000 commercial plastic grinder)