Currently, we are ingesting a complete list of subreddits from Reddit. There are near 10 million subreddits in existence with most of them being user subreddits. You can test out the new endpoint to search for subreddits using the API that is in beta (beta.pushshift.io). Here is an example of searching for the term "science" and sorting by the number of subscribers:

https://beta.pushshift.io/reddit/subreddit/search/?sort=subscribers:desc&q=science&pretty&filter=display_name,title,header_title,subscribers,subreddit_type,public_description

This will show subreddits with the term "science" appearing in either the title, header_title, description or public_description.

You can sort the results by number of subscribers or created_utc (epoch time). You can also see when new subreddits were created and do aggregations on specific fields.

Let me know if you have any questions or suggestions. Remember, until it goes live in 2018, it is in beta and may go down for maintenance and be otherwise a bit quirky. :)