Nationals can provide work but be careful... most pay peanuts, most pay slow, most want tons of pictures and many will squeeze you with charge backs!
check out http://www.trashoutstartup.com for their ebook which would give you a pretty good idea
I would recommend that you talk to any real estate agent that has REO listings. From there attempt to learn as much about the trash out assignment process as possible and work to make contacts with the people who do the assigning. If you find that the agents are the ones in that area that make the decisions, work to build relationships with them. If the Asset Management companies are assigning the work, then work to build relationships with asset managers at the companies who are doing a lot of work in your area. Good Luck!!
Coldwell Banker Burnet
licensed MN Real Estate Broker