What is the best way to find Wordpress maintenance clients?

Create targeted Ads. What you must realize is that when you are in business, any business, you are in fact in marketing. Everything you do should be with focus on how to market it.

Marketing in itself now a days is engaging your potential users. Create targeted messages for certain situations you see a 'client' needing help with. Then promote that to the best possible sites/events/meet ups/ etc where this customers with this certain issue might gather.
Now, keep in mind that each platform has its code of conduct. Twitter for example is good for sharing info, links and other media. Instagram is best for imagery and hashtags that target certain needs. Not all your services should be promoted on Instagram. Facebook is for leisure so the content you share there should be minimal and native. Native means no long posts, no ads posting links and instead more posts that have minimal text within an image, short descriptions (call to actions) the content used should again be targeted. Have maybe the same image but with variations of messages, and dedicated towards each need you can think of.

Likes mean nothing, if you do get paid ads, have those ads link targeted single page content that have one specific goal: get emails, get shares, get downloads, buy a product, book a call, etc.

Is hard work but if you see it as a strategic approach to every effort you do it will pay back. :)

answer questions that they have via blog posts or videos.

I have found that the best WordPress maintenance clients are your current clients using WordPress. Build their site and then maintain it. Clients you host their website for, maintain it.