S., congratulations on your twins! Mine will be five months old this week. (I also have a 2.5-year-old.)
After a fairly extensive and pretty lengthy search, I finally found a nanny from the NDMOTC (North Dallas Mother of Twins Club) bulletin board. I got most of my leads from networking: talking to people at my toddler's MDO program, the pediatrician's office, my neighborhood ECPTA, my husband's office, my playgroup... pretty much everyone I could think of. I also posted a flyer at the Dallas seminary (but got no responses) and checked craigslist-dfw. I did check with a couple of agencies but never signed up with one.
Including taxes, we're paying her about $11.50/hr. for a 40-hour workweek. In my search, I found that that was in the middle- to low-end of the local salary range... but I'm a stay-at-home mom and couldn't rationalize paying someone $15/hr. for someone who's basically a mother's helper. But your situation may be different.
I hope that helps! Best of luck to you.