If you are in Edmonton, Johnston's Industrial Plastics sells it by the 4'x8' sheet. You probably don't need so much, so you may want to find others to split it up (I'm willing to take a 45"x30" chunck is available!). IIRC, it was about $120 for the whole sheet...that is IF I remember correctly
Usually, any plastics shops carry this stuff. Its also called HDPE (High Density Poly Ethelene) but I'm sure there are many other names for this stuff.