Acceptance rates are used to measure the relative quality of a journal, similar to acceptance rates for universities. The lower the rate, the more 'exclusive' the journal, which presumably means higher-quality. This is often used when impact factor or other measures are unavailable.
However, not many resources exist to help you find an acceptance rate! Sources include:
1) Cabell's - published rates for some disciplines (but is not comprehensive within those disciplines). Disciplines: Business, Education, Psychology, Computer Science and Health.
2) SciRev - crowdsourced information about journal submission experiences. May also include acceptance rates and other relevant information.
2) Journal website - some journals publicly advertise their acceptance rate.
3) Direct call to the journal - often, this is the only way to get a current acceptance rate for a journal.
Input a title, keywords, abstract, etc., and EndNote will show you the most relevant journals indexed in Web of Science. To access you need to register for a (free) EndNote Web account, then select "Match"