There are several sources online that rank math journals by impact factor (and, this comes from a professor's webpage) or journal citation reports. However, it is important to realize that impact factor is a highly unstable metric, as discussed here.
Thomson Reuters has a list ranking by JCR, which appears to now be behind a paywall called InCites (though you might be able to access it through your library). If I remember correctly there was also an option to rank by impact factor.
The Institute for Scientific Information (ISI) is another source that maintains a database. And there's Scimago mentioned by Yemon. And you can look up impact factors over the years at bioxbio.com.
Another ranking by impact factor was SciJournal.org, but their website also appears to be down. A general way to solve the issue of disappearing website is the WayBack machine. For SciJournal, here is a recent snapshot from 2020. For the Australian list, here is a July, 2020 snapshot. In the same spirit as the Australian list, I am aware of a Chilean list that breaks journals into categories such as "Muy Buena," "Buena," and "aceptable" (I'm not sure if this is actually the third category). I've heard that this breakdown is not super well-regarded.
Lastly, an alternative to impact factor, that I've read about but have not yet looked into, is scite.