I assume you have drilled discs ?, if so and the cracking is just tiny cracks radiating a few mm from the drillings don't worry, no need for new ones, it's when they split with a split all the way to the outside edge of the disc and 2mm wide gap you need to worry.
Look on rallye design as the his spec are only std discs of a ordinary car/van etc with the centre bell part cut out, then drilled to suit the alloy bells, so find the diametre and look on rallye design, they are two main bolt patterns with either 8 or 12 bolts.
tabetha