You have two issues: deciding who to approach and then what to do to make them support you. I'll focus mostly on the second part.
But first, the upper level students in your university will have a pretty good idea who the best teachers are. In some (smaller) places that might be all you can hope for. But you can also ask any professor, including your academic advisor, about their own research and a general question about who on the faculty does a lot of effective research. For math, however, you might need to specify a field within math so as to match your own skills.
And a combination of the two approaches (ask students and faculty) might be best. My further suggestions will depend on finding someone who is willing to do actual research with undergraduates. Finding a superstar who won't work with you will bring no benefit. And some such people are too focused on their now research, which is probably above your level anyway, to provide any help.
But once you find someone who does interesting research in a field that also interests you and has some track record of working independently with undergraduate students, approach them for an independent study research project that they would be willing to guide. You probably don't need a specific topic, but a field (algebra, topology, ...) would probably be essential.
Then, spend a lot of effort on that project. If you do a good job of it, then they will be impressed and will probably be willing to write you good letters of recommendation for further study. But it is your work than matters more than the (international) reputation of the letter writer.
If you can get a publication along the way, even a joint publication, even a modest publication, all the better, but as an undergrad this isn't essential.