I ended up signing up for a free month at Sittercity.com and then canceling after the job was done.
I offered $25/hour as I thought a nice high rate would mean lots of applicants. I was right about that. Also, in my head, a high hourly rate meant I wouldn't need to pay for parking. But silly me didn't specify that in the ad. So I chose a sitter, confirmed the job (3 hours, total $75) then got a message from the sitter the day before letting me know her parking would be $15. Lesson learned, need to be clearer. I think I would have had plenty of takers for $75 excl. parking but as I didn't specify, and as I don't know the convention (but it doesn't seem crazy to think sitter would pay for parking) I had an unexpected additional charge.
Turns out the concierge nanny service would only have been about $25 more at the end of the day, and that would have been a little less stressful.