This principle is not difficult to improvise from and one could also use a metal barrel which would absorb heat better, making thermophilic microrganisms break down your shit quicker.
This one looks fixed to the barrel and rotates on the sawhorse as a baring. Mine has the pole fixed and has a track bearing (for a garage door) at each end of the barrel. If using either principle be sure to make it a bit higher than the one in the picture so you can dump the material into a wheelbarrow. Note the axle has drainage holes (mines metal not pvc, so it doesn’t) so you can get some compost tea.
Cutting the door can be difficult in the plastic as the heat will make it warp over time. When doing so, be sure to use a strong latch, or the weight of the soil can pop that shit open when turning. Mine uses two. and has weatherstripping around it to keep a seal. With metal, a cutting torch should make the job nice and clean.
Also, one can put a fire under it if it’s metal to sterilize that shit.
All in all it’s simple but often overlooked. As to the location of barrels, one can go to places the use things like tomato paste (which my barrel contained) and the like. DO NOT use anything petroleum based or if you do, wash out thoroughly.
Catering to your plant(s) of choice:
One can purchase cheap topsoil from the dollar store to amend or obtain topsoil from a loamy, fertile place (this contains more potential pathogens; do sterilize for quality specimens). Once you have this in the barrel, mix topsoil filling 1/3 the volume of the barrel with a 5 gallon bucket of cow manure, some phosphates, some potassium salts, and most importantly, a good helping of seaweed. This, so I have read contains many micronutrients and can help prevent a myriad of disorders.
If the organisms like organic matter ( i.e. sylvan plants, plains plants, Cannabis) leave it as is. But if they (xeric plants) like well draining, sandy soils, you ought filter out the larger portions of bark and wood. I would not use much organic matter in slow-growing cacti mixes. The resulting mix can be used as a base to be lightly or moderately, depending on the plant’s tastes, added with a mix of crushed red lava rock, perlite, and sand. Blueberries like a fairly well draining medium for instance.