If you are buying canvas as an artist intending to sell work, you should look for the best canvas available. Belgian linen canvas is known to be the highest quality, and will resist deterioration and tearing.
Buy canvas that is stretched on at least 2" thick, kiln-dried stretcher bars. No staples should be visible on the front or back of the canvas.
If you are buying primed canvas, make sure the particular canvas you are buying is appropriate for the type of paint you plan on using. Some canvases are designed to best fasten certain types of paint or other media.
If you are buying canvas to learn to paint or are painting as a hobby, you do not need to buy the most expensive archival-quality canvas. Look for student-grade canvas, which is considerably cheaper.
Do not buy canvas with staples on the side unless you are sure that you will be framing the canvas or will not be displaying it at all. Canvas with stapled edges is okay for practicing and learning, but is usually low quality and not meant to be displayed.
Purchase canvas with 1" thick stretcher bars. This will save you a great deal of money over artist-grade canvas. It is best to buy canvas stretched on kiln-dried stretcher bars, as this type of canvas tends to hold up the longest.
Stretched canvas can be purchased online through a variety of stores. Online stores like PearlPaint.com and ArtSupplies.com offer a variety of stretched canvases and other art supplies for both students and artists.
Local retail stores like Aaron Brothers and Dick Blick Art Materials carry stretched canvases for both students and professional artists. Some retail stores have special sales at various times throughout the year on stretched canvas and other art supplies.
There are some small businesses that specialize in building high quality stretched canvases for artists. These canvases tend to be quite expensive, but are built to the highest specifications, and can be built in custom sizes and shapes. Look in your local phone book or in an online directory to see if there is anyone in your area who builds custom canvases.