If you are going to be stretching your own canvases there are a number of options, from the local wood yard to the College MDF strtechers. Personally I find MDF too heavy for large canvases and not only that it is not as stable as you my think. MDF will bend if your canvas is a bit tight.
One of the cheapest places I have found for stretchers is www.stretcherbars-uk.com They have a wide range of sizes and qualities at very reasonable prices. Certainly cheaper than going to the local wood yard.
