If you're a part of YC or other similar investor/tech networks, often those are very strong referral networks.
Beyond that, there are various niche job boards and sites like https://www.fractionaljobs.io/, https://www.hirefraction.com/, marketerhire.com depending on the type of work you do.
Sites like upwork/toptal can be good but often are a race to the bottom.
Relevant: I started a newsletter a little while back exploring this space for tech workers