you can either google or use RSS Submitter software ( to ping ) and manually register where needed. there are some 80+ rss directories included ( the submitter/ping too costs less than us$100 - thats a bargain compared to the benefits of feed submissions/ping or you may find on the page howto RSS a few dozen RSS directories - they all have been free when I used them last time. Just scroll down the page to see all directories.
Even if you don't buy any RSS submitter software there are a lot of FREE online directories you can submit your RSS link to. See toprankblog.com/rss-blog-directories
Here is the link which is having the list of free rss feed submission directories. http://www.rss-specifications.com/rss-submission.htm