You can probably find an Asian market which will order you the black raw sesame seeds in bulk. I freeze any extra flax or sesame seeds I use to keep them fresher until use. Be careful that you are not getting processed sesame seeds with the hull removed. It may take some effort to find a propietor of an Asian market who can speak English well enough to get you what you need.
You could also find a source on the internet. Look for one that offers a phone number to make sure the seeds are raw and not processed in any way. Jill