There are tons of websites such as Etsy, Alibaba, Ebay that sell septum rings online or you can easily buy them from a department store like Forever 21 and H&M.
So many people (especially men) have told me that girls that wear septum rings look like cows and honestly I have no clue what they are talking about. I think the key is to find the right size for your nose so if you have a larger nose then go for a smaller ring. If you are wearing a septum ring then I would recommend to keep all your other jewellery lighter so, I wouldn't add a nose ring as well. Also, it depends on the "look" you are going for. Personally, I love how a septum ring looks with a really nice bold, matte lip!
I didn't wear my septum ring for too long because I started wearing a nose ring and it was way too much jewellery around my nose. Also, my mother told me she was going to disown me if I kept wearing it.....boo-hoo!
Like I said I absolutely love it and here are some pics below to help inspire you to choose the right one for you.