What are good sources of humus?
Canadian "sphagnum peat moss" is an excellent source of humus. However, avoid the use of "native" sedge peats. They cause more problems than they do good and most of it is just silt.
Sphagnum peat is a special case of humus. As it comes to the user it is essentially pathogen free and need not be composted. The Colorado State University Agriculture Extension service recommends the application of 3.5 cubic yards (uncompressed) of sphagnum peat per 1,000 square feet of soil tilled to a depth of six inches annually. If the tilling depth is greater, then proportionally more peat should be added.
Adding much larger quantities of peat, in one year, can result in excessive retention of salts from spent fertilizers. The peat may retain water with the salts. Very rapid surface drying may evaporate the water before it has time to drain away, leaving some salt on top of your soil rather than draining away with excess water.
As sphagnum peat is decomposed by micro organisms they burn large amounts of nitrogen which must be replaced. Adding 1/4 ounce of "Nitrogen" or 3/4 ounce 34 N fertilizer per square yard of soil will compensate.
Another important attribute of sphagnum peat moss is that it reduces the pH of soils to which it is added. For that reason it is particularly useful as an amendment to alkaline soils. The Reader's Digest "Illustrated Guide to Gardening" claims that 50 pounds of peat moss added to 1,000 square feet of garden area will reduce the pH of the soil by about 1 point. I have found that it takes almost 100 pounds to reduce the pH by a point. Of course, how effective a substance is at reducing pH depends on where on the pH scale the soil is rated at the outset. The rate of pH reduction is not linear.
Uncompressed
Sphagnum Peat Moss is sold by the "compressed" cubic foot. A common size is 3.8 cubic feet compressed. When uncompressed, that yields about 5.7 cubic feet.