I think you're asking a question that can't really be answered except on a case-by-case basis, so the answer is going to be completely different from one person to the next.

Comixology was one of the highest grossing apps on the ios store for 2 or 3 years in a row, until Amazon bought them out and removed the ability for in-app purchases.

But even that isn't a true indicator of how well indie comics are doing, since they sell everything from indie to major publishers.

Maybe try researching some indie comics you're familiar with, get an idea of how popular they are and find out how they got their audience.

Look at the top indie comics on comixology and other sites, and see if you can find where they originated. Example, one of the comics on their indie top sellers list had a kickstarter campaign in 2015 that netted over $160K in backer funds prior to publication. I found that in 5 minutes by picking a title at random and doing a google search. Kickstarter has been pretty good for at least some indie comics. So has patreon.

Might also try joining a forum dedicated to indie comics and ask people there, since their info would probably be more accurate than what you would find here.