I don't currently have an agency and not many reliable agencies are based in my town, so I was wondering if submitting to nationwide castings might be a good idea. Is there a place I can subscribe to to find out about the good castings?

Just bumping this post back up.

Are nationwide castings even common enough for me to be asking this??

They are not common. Have you read any of the books on the business suggested in the reading section? All of your questions have been dealt with there.