I am looking for recommendations for a nice, affordable B&B for Sept.
I was looking for a B&B for Sept. but had dufficulty finding availability at any of the usual places recommended by Rick. I did find several available at cross-pollinate.com, the service offered by the American transplant couple who run the Behive. The Beehive is also an often recommended option. Their website is the-beehive.com. Also, you might try santasusanna.org if staying at a convent is of interest to you. That is what we chose - excellent value, clean, comfortable, safe, and an interesting experience. Good luck!
Nicholas Inn is tough to get into (but looks nice). We're staying at the All'Obelisco near the Spanish Steps. I would use a search engine like on Orbitz or Travelocity to see. Usually you'll get a cheaper rate by contacting the B&B directly through email on their site (and paying in cash when you get there). Good luck!
I suggest B&B Giovy near the Termini Station in Rome. I plan to stay there in November, so I don't have any personal experience yet. However, the reviews on tripadvisor.com are quite positive. See the links below. For other places you could do some searches on venere.com or tripadvisor.com.