Idk how and Serebii says it's available.
No other way because it's unobtainable in B/W2 and that's when HA's were released. So unless you have a Friend Safari with Mr.Mime accessible, you're out of luck.
Hope I helped!
-It's a secret!
You can only get them through a Horde Battle in Reflection Cave in X&Y. You'll need to scout each and every single Mime Jr. in the horde until you find a Technician one, knock out all but the Technician Mime Jr, and catch it. Level it up to get yourself a Mr. Mime.