When you've just got to have a sitter, call Best Sitters. I am sure I am one of their worst customers - forever calling at the last minute (at least once, quite literally) because I forgot we had an event or because I thought I could get away on the 'cheap' with a neighborhood kid who later bailed on me. Best Sitters has not ever let me down. They are always able to find me somebody and I nearly always like who they send.
I feel they're pricey (hourly fee, 4-hr minimum, plus agency fee & I always give the sitter a tip too) but worthwhile headache prevention and they serve most of the Seattle metro area, including the eastside.
Ideally, I suggest making a list of preferred sitters (once you get to know some of them) and call early enough to request them specifically. At a minimum, it's imperative to call way in advance for Valentine's Day and Holiday Party nights out as these are their busiest times of the year.
In any case, it sure beats calling ten teenagers hoping that you'll find just one who doesn't have a date, a football game to attend, or a curfew too early to suit your needs.