Any of the sheeting material (Corian, Silestone, etc.) manufacturers make sheets. Some are packaged into kits. You can make the whole thing monolithic if you wish, but building a tiled shower, especially one with some of the newer materials (Kerdi or Wedi come to mind) would also work well.
Keep in mind that your drain on a tub is likely 1.5", and a shower requires 2". Moving it to the center of your shower is the preferred location so that the slope is more even all around the shower.
If you want to consider tiled, consider checking out www.johnbridge.com.