Not sure if you have an advertising budget, but you may want to check out Potomac Tech Wire. It does a weekly "job blast" where you can post your opening. Related Startup Digest has a DC group. You may be able to synch up with them to post your opening or find avenues to locate and engage salespeople.
Also, as a quick hack, you may want to update your LinkedIn profile message to state "ComapnyX is Hiring Salespeople."
Otherwise, I've found when hiring sales people, a lot of word of mouth or looking at who is working firms in your sector (using LinkedIn) and seeing if any of them are interested in a new opportunity with you.