Ancient Forest Humus. Fresh earthworm castings, and\or back-yard or commercial compost (e.g. BioMax sphagnum\kelp or shrimp compost). Cow manure from the hardware store tends to be loaded with sand and varying degrees of undecayed debris. This probably depends on brand (Black Kow is one of the better ones I think), but I personally still wouldn't use it due to the available alternatives which I perceive as more favorable.
Ancient Forest can contain small pebbles and chunks or materials\twigs, but these usually disintegrate easily. I just take out any pebbles I catch and use about a cup per gallon, which I have 'processed' (no clumps or anything). AF has thousands upon thousands of species of microbes. Molasses is the only other ingredient really required to get exploding bacterial populations. With AF the foam starts almost instantaneously and promptly thickens up for the duration of the brew.
Guano can also be used in AACT with the compost\castings\humus, or even as an inoculant by itself (with molasses).