How to find the acceptance rate for a certain conferrence in a certain field?
The only way to know is to ask them, i.e., find the contact of one of the editors, and politely ask him/her what was the acceptance rate for that particular year.

There are multiple lists available on the internet:


http://haofengjia.weebly.com/computer-science.html 
http://www.lamsade.dauphine.fr/~sikora/ratio/confs.php
http://www.cs.ucsb.edu/~almeroth/conf/stats/
http://kodu.ut.ee/~dumas/LiquidPub/ConfStats/
https://lipn.univ-paris13.fr/~andre/conferences.php
http://web.engr.illinois.edu/~taoxie/seconferences.htm


as well as the links they provide.

But they are as reliable as your trust in their authors: there is no guarantee whatsoever, in most of the cases, that their sources is trustworthy, and they are very scattered most of the time.

Related questions:


Conference ranking in computer science
Is there any world-wide ranking of conferences/journals?