As with many things these days, the place for finding sessions is on Facebook - sorry if you don't do Facebook.
There is a group/page called Edinburgh Trad Music and Session Hub: you can post a request for info on there, or go to the "Featured" tab, which has a spreadsheet of known regular sessions on it.
And for Glasgow there's a similar page called The Glasgow Jam Sessions.
And, of course, there are many other places in between that hold regular sessions!