WE've used Parents in a Pinch from my job, which is different.
IT doesn't sound like you have the time to do this, but we tried them out in advance before we really needed them.
One idea is to have someone on a day when you work at home. If you like that person, request them again the next time. Do this a couple of times.
But I found that they varied in quality but were never totally bad. The worst temp I ever got was from my nanny agency and I paid for it myself. Live and learn.