A real basic manual way to get a list for starters would be to go on your phone, find one high quality relevant source, follow it and see the suggested users come up and then go through that list and note down all the relevant ones that seem good. And then ones that seem particularly good, you click on them and do the same thing and follow them, find suggested users for them and write them down. This can also be done through certain softwares, but since you’re only doing some accounts you could probably achieve this manually with a little effort. And from there, hopefully your software has a way of giving you data on who has followed back from each source so that you know what works and what does not. If not, do your best to work out what sources are working.
Also, if your software allows for following by hashtag, then for some niches that might actually work fine. E.g. for organic food, people who make the effort to put #organicfood or things even more specific probably have a decent interest in that field. Or for the other ones, stuff like #dentalhygienist or #dentalassistant would be relevant because not many people would just put that on their posts unless it’s relevant.