I know someone whose word I trust has got one from a Raid.
I've herd the same thing that the odds are better from a raid but but nothing conclusive also.
From what I can work out going back through the web. Shiny Magikarp was released into the game March 24th 2017.
I started a log counting every single Magikarp I clicked on attempting to catch April 25th 2017.
It was 143 days and 564 Magikarp to get my 1st Shiny
It was 9 days and 77 Magikarp after #1 to get #2
I'm now 10 days and 285 Magikarp since #2 looking for #3.
I do every Single Magikarp Raid I come across time permitting. So far its 37 Raids in Total none Shiny.