You can post on this website, in the "DC- Nanny Search" forum. You can try sittercity.com or care.com. Or I would recommend trying one of the local colleges if you only want a limited number of hours per week. I know American University has a job board for babysitting -- www.jobs.ausg.org. The other colleges might have websites, too, I just don't know what they are.
By the way, I don't think you're really looking for a mother's helper. That's typically a younger person (high schooler) who comes in to play with the kids while the mom is nearby. I think you're looking for a part-time nanny or babysitter, especially since you're talking about caring for an infant.
Hope this helps!