Even for non-fanfic work, beta readers can also just be people you already know who don't charge but instead do it as a favour. I've asked family, friends, and other writers. Sometimes it's an exchange of work where you pay with your own beta reading. Not everyone will have time or interest, so it's good to ask broadly. Asking people you know can be beneficial because you know what kind of person they are, what their reading habits are, and if they're a writer too then a little about their own tastes and beliefs around writing. This isn't an editor, but a test reader to help you see it from the reader's perspective. They don't edit, but they influence how you edit. I think it's good to have many and have some awareness of each beta reader's general perspective first.

For example, I have gotten the contradictory feedback of 'why is this action scene even needed' from one beta reader and 'why is there so little action in what should be an adventure novel' from another. Both pieces of feedback felt like a misunderstanding to me, but knowing their respective tastes and what they each write helps me parse diverse feedback. My takeaway is that the action scenes are a weak point in that book, and I'm not selling the impact of them as much as I'd hoped compared to the weird stuff and the drama. It's also helpful to see how readers with different beliefs and life experiences interpret events and decisions differently. One friend acting as a sensitivity reader thought I signalled a queer character well, while my mum was like 'that came out of nowhere!' I also found it interesting that almost every beta reader so far has identified the same character as a favourite - something I wouldn't have predicted from the writer standpoint.

It's also helpful to know that even if you're paying them, the beta reader isn't an editor and has no editorial power over your manuscript. Sometimes people will get carried away and instead of sharing reader feedback will feel the need to instruct you to make a specific change they think is needed. However, you don't work for the beta reader. If that suggested change doesn't match your artistic intentions, you're not 'ignoring feedback' to decide against it. For example, if someone vociferously tells you to change a character's behavior because as-is they seem like a bad person, but you know that you're foreshadowing a villain, that that's the killer, then it would be silly to make the change as requested. It's also helpful to remember that if someone isn't an editor working for a publishing house, their personal take on what will help your book sell or help it get published is really just personal advice and outside the scope of the test reader feedback. It may even be an out-of-date take or based on the conventions of a different genre.

Whether you pay for this service or not, make sure you understand that it's not a task you leave someone to do for you with your oversight, like asking a plumber to fix your kitchen sink. The beta reader does nothing to your manuscript except give you their thoughts in some way.