Oh, not that kind of contract work? Well then...My first stop when hiring extra help is a tech-focused temp agency, here in the SF Bay Area I've had outstanding luck with Robert Half. Their recruiters speak tech and understand my company's needs. Temps I've used/interviewed really know their stuff and have been happy with the arrangement (pay, flexibility, possibility of benefits through the temp agency). As far as I've experienced they treat customers and their temps fairly as they understand the value of all involved. I'm contacting them this week for a database setup gig. Downside is buyout cost if we really like the temp and want to hire.
For finding contractors, schmoozing at conferences and personal network have been the best, but that takes time. Have spec'ed things out on Upwork and had good work done (I've never based contractor selection on low price).
Best of luck on your search!