They were a lot more popular in the 90s and early aughts when Linux was still new and people were looking for community. I usually would find out about them through local Linux User Groups (LUGs) at the time [1].
I remember a Linuxfest in 2002 in Southern Ohio (not OLF) and John "Maddog" Hall was also a keynote speaker. Looking at his speaking schedule is probably a good way to find similar events. My most distinct memory of it was someone had a table of NeXT computers setup to use.
Along with OLF, SCaLE (Southern California Linux Expo) [2] is another large Linuxfest type event. Much more of a technical and casual vibe, and satisfies personally instead if professionally. I did a GPG key signing party at it once and it was a nerdy and fun as it sounds.