I have had a few subs. Word of mouth, and advertising around town (library, preschool, etc.) worked for me.
The main thing I learned was to pay WELL. This will attract higher quality canidates. I prefer that my sub have child care/teaching experience - my current sub is a former K teacher who left the work force to be home with her son (now in school). She is WONDERFUL with the kids. Always has lots of ideas, not afraid to pull out the arts & crafts/sensory or get them outside.
I also try to keep it very formal - there will be times I have to address issues and I've learned the hard way that if I hire a friend I'm likely to let things go and just get upset about it
In my state they must be fingerprinted, undergo a background check and have current CPR/First Aid.