Yes, but it is spendy. Go to a local sign shop and ask them if they will order you a sheet of something called polymetal. It is a plastic core with an aluminum skin. It's available in lots of colors, and takes paint well if you want to paint it. You can get it in 1/4" or 1'8". It cuts easily with a router or a saw, and is rated for outdoor use.
If that doesn't work, they should also be able to get you PVC sheets (paylight is the most common brand) - white or colored as well. It is also easy to work with, but is NOT rated for outdoor use. It tends to want to warp a little with time. If you have a frame to connect it to, it should do just fine.