I sell locally but want to expand nationwide
I would recommend that online. Write articles that would magnetize your kind of target market and submit them to article directories.
Trade exhibition are great, especially if you can locate product anywhere in the country, be sure you have representation there.
Send out press releases and story ideas to product editors and writers at newspapers and magazines around the country. Look for publications with new product sections and see who writes them. Target those people with your press releases and consider sending them a small sample of the product.
You should also look for bloggers and websites that focus on the product market. Send them articles about the benefits of the product and be sure your contact info and website link is included in the resource box.
You may also want to try affiliate programs. There’s tons of information out there to assist you.