Unless you know a niche Reddit topic really well already, you are likely going to have to do some research to find the Subreddits out there. Subreddit Stats is a great site to start! You will get the statistics on subreddits including the growth over time and subscriber count. PLUS this tool is perfect for Reddit discussion and keyword research by monitoring the number of times a certain word or topics has been discussed already within a Subreddit.
The Daily Carnage