Unfortunately there are no US emabassies or consulates in the US so you have to make do with the less efficient US Passport offices.

To wit:
* WASHINGTON Passport Agency
1111 19th Street, N.W.
First Floor, Sidewalk Level
Washington, D.C. 20036

Hours: 8:00 a.m. - 3:00 p.m., local time, M-F, excluding Federal holidays
Automated Appointment Number: 1 (877) 487-2778

check online at travel.state.gov to see if they will let you get the pages added same day. I assume that you are about to apply for a visa that requires some number of clean pages? (China, Brazil, wherever) If you are doing something like visiting the UK or europe they will just keep stamping full pages.

Another option is to use a passport service. This is a good option if someone else is footing the bill (ie your company). These people often take up all the appointment slots at the passport agencies.