I totally get it that buying a paid promotion can be hard. So, there are a few options, not quite as reliable as Fussy Librarian, but you could try:
1. Fiverr. For $5 to $25 dollars you could pay for the promo. It’s much less than some of the big independent email lists. However, you’ll really need to see what their channels are for your books. With my devotionals, I have tried this route and they are pretty hit and miss.
2. Try friends in writing groups to help with a social media push. This could make it hard to get the Amazon algorithm working in your favor because you need it to capture your target audience and friends aren’t always it.
3. Try writing a tone of articles based on sections of your books with links to your books at the end. This can drive some traffic, but isn’t always reliable.

I hope this helps!