I work 3 days a week too, and we have a nanny.
I've been back at work for 3 years (just started maternity leave again, awaiting arrival of DC2), and we have had two nannies.
We got the firts through an agency, which was local to where we live. I just did a google search for nanny agencies in our area. We basically gave a job decsription of hours to be worked, where we lived, what we expected, what we were willing to pay etc. And they sent us a load of CVs and we interviewed a few nannies. Out of those we interviewed, we made one nanny an offer, which she accepted.
She was pregnant within a year, and stayed with us for 18 months in total before she went on maternity leave. We got our next (she is our current) nanny by word of mouth - our nanny had a circle of nanny friends who knew that she was pregnant, and one of them wanted to change jobs and so she came to us to cover the maternity period. And as our first nanny didn't return to the job, she stayed with us.
Having had one nanny through and agency and another through word of mouth, I would recommend the word of mouth approach every time. Obviously though, its hard to find someone by word of mouth when you don't know nannies yourself and don't have access to that circle. Do you have any friends with nannies?
There is no easy way to find a nanny who suits your family needs though. Our first nanny had marvellous references (which we followed up), and we were happy with her, but now that we have had a second nanny, I really see the difference between a good nanny and a great nanny. A good nanny with look after your children well. A great nanny will do this, and fit in with your family needs and dynamic well too. Honestly, I can't imagine that I would have had another DC if I didn't know that we had our nanny to rely on when I do go back to work.