Boy, that is an easy one.....plently of cow manure on this board!
Gen, I would recommend a blend of cow manure with another soil additive; too much manure may burn your plants.
If you are looking for a large quantity, Natural Gardener and several other places can deliver or you can get a pickup truck load, usually a half a yard will fit into a pick up bed.
I have gardened for years and love it for the stress management as well as the benefit of eating out of your own backyard; and now is the time to begin preparing your soil for the spring planting.
A good mix of cow manure, compost, and light sand will work very well for most Austin gardens. I do raised bed gardens so as not to deal with the weeds and heavy clay soil around my house.