On Varick (or it might be Hudson) just south of West Houston Street, is a US Passport Center. They will give you a 24-page insert to add to your existing 24 page passport. I don't know if you can just show up. They have an appointment system that you can access via telephone. (Google it, is your best bet).

If you are getting close to your 10 year renewal date, you may just want to get a new passport. When you get a new passport, you can request a 48 page passport instead of the standard one. Since there is a fee to do either one, you may just want to get a new passport issued, which will probably be more expensive in the short run, but save you money in the long run from not having to pay 2 different fees at different times.