There are probably loads of suitable images to lift straight from the internet. Google the image 'dartboard' - you will find several that you can then just map onto a cylinder (which you can create in Blitz).
Failing that, Google 'dartboard 3d model free' (or similar) - might bring up some free pre-made ones.
Failing that, Google 'dartboard 3d model free' (or similar) - might bring up some free pre-made ones.