ATTQOTD: For local races, we have something called Little Red Hen Productions. If I'm looking for something specific, I use Active or Runningintheusa as well.
I noticed last year that 5k runs have become really popular while 10ks were less frequent. I've actually skipped 2 races this year because they were 5k and we needed more miles for our long run Saturdays.
I find 10k's lacking as well, and many of the 10k's that do exist are just two laps of the 5k course A few times I've signed up for a 5k, but arrived at the site early enough to get in my targeted mileage for the day, minus 3 miles. By the time I get to the start of the 5k, I'm all warmed up and ready to race. The downside is if your local 5k race organizer isn't all that organized and the start is delayed. Then you are just standing there tightening up.