Rescue Pearl is a very good source. Will also cut you rosettes and purfling strips.
If you want serious amouts and a serious selection go to the maker, RT Research http://rtresearch.com/trustone but you will need to cut it from blocks yourself. Pretty sure this is where companies such as Masecraft and Rescue Pearl buy from. If you don't have a good way to cut it like a diamond wetsaw or thin kerfed bandsaw blade for the softer stuff it isn't worth buying it in blocks even though the cost is about 25% of what others sell cut pieces for. You really want a well set up lapadary saw with a thin kerf blade so you don't get much waste.