Some dealers mix pieces of new wood with the old. Wood cut to expectations. Those shapes may be what you want. But some connoisseurs, like the UVA's Howard, feel that with heart pine in particular, only quarter sawing will play up the tighter texture of the antique wood. Because so many of these dealers do their own milling, they can accommodate special orders including boards a foot wide or 16' long, or unusual widths or thicknesses.
Many dealers have well-illustrated web sites, which is a fun way to begin exploring.
Many will send you a photo. But cameras can lie, so ask to see some samples of the actual flooring. And you may want to know the source of reclaimed lumber for reasons other than gleaning a colorful anecdote. Drake researches the buildings' histories to make sure they weren't the site of chemical-intensive industries such as tanning. I wouldn't feel comfortable with that for my workers or my customers.