The thing that makes Pyure unique and a good choice for fruit flavors is that it contains, like you said, water and stevia, but it uses apple cider vinegar and citric acid as preservatives. Both of those are common additives you’ll see people talk about adding into fruit and sometimes tobacco mixes. I think that’s why everyone likes the Pyure so much is because you don’t have to add anything else, it’s an all-in-one kind of deal. A lot of liquid stevia brands do have ‘natural flavors’ listed in the ingredients or things like sodium benzoate or potassium sorbate as preservatives. People tend to try and avoid those.
I’ve not done extensive research, but I haven’t come across another brand that makes stevia like Pyure. If you can’t get it or it’s unreasonably expensive, I would look for a stevia extract that is organic and only contains water, stevia, maybe alcohol since that’s a pretty common thing in e-liquid, and/or glycerine. Try to avoid ones with weird things.