Look for college students on care.com or post at your local college. My coworker found a college student on care.command she’s very reliable. I don’t know about less than $20/ hour. She charges $22/hour but she’s not in school in the evenings and weekends
I think under $20 is not likely, unfortunately. It's a lot to pay, and I know the struggle. 😩 I managed to find babysitters in the $20-$25 range, posting through parent FB pages and sitter city. If you're upfront about what you can pay that helps to narrow the search.
When you find one let me know I’m looking too