Soils are commonly available either bagged or in bulk. Bulk topsoil is generally a native soil taken from the surface, screened and sold in truckloads. You can look at Kijiji, yellow pages or newspaper ads to find bulk topsoil suppliers.
Big bags of soil that contain 1 cubic yard (length X width X depth) are now available at landscape supply companies. If you are doing a large project yourself and can only deal with so much soil at a time, this is a good option.
Topsoil is a Screened Loam that is typically used under sod. It provides a stable base and the nutrients that lawn needs to thrive. Topsoil is the uppermost part of soil ranging from 3 to 10 inches. Topsoil is good for use under lawns as it sets up firmly to hold up under foot traffic.
Garden mix is a 3:1:1 ratio of soil, peat moss and sand and is highly recommended for shrub and perennial beds. This nutrient rich soil provides good drainage while retaining a moderate amount of moisture under dry conditions. It stays softer and more workable in the shrub beds while providing a slow release of nutrients over time. It also creates good air circulation around root systems.
One of the most important things you can do to make your project sustainable is to use the proper type of soil for your project. Topsoil eventually turns very hard (like concrete) in shrubs beds, usually contains an abundance of weed seeds and does not hold moisture well. Garden mix does not set up firmly enough to be used under lawn.