The dress is by Jovani, a high-end prom dress maker. I've always been a huge fan of Jovani, as each one of their dresses is absolutely gorgeous and one-of-a-kind. The brand is popular among celebrities; the dresses have been worn to red carpet events, concerts and even in magazines. Selena's dress is no longer available, but I saw a dress of a similar style on Miss Selfridge's website:
The outfit isn't complete without the diamond-shaped drop earrings. Here is a pair from Topshop:
For the dress, visit Miss Selfridge and click here for the earrings. To check out the Jovani dress collection and to find out more about the designer, click here.