Artists' canvasses can be bought from most art and craft shops and come in many sizes, so it'll be easy to find one (or several) which fit the bill for this project. Try a few in several sizes for a modern and fun look - spray each one in a different colour and finish off with stickers which tie into your theme! You can find them in craft shops, or look for friezes in the wallpaper section of DIY stores and cut out the details.