If you're talking the Roku device, then it's available at many retailers, in-store and online, from Walmart to Amazon to your local hobby electronics store.
If you're talking the Roku TV service, Roku is a company that offers a way to host and transmit streaming content to end consumers. Google Chromecast, Apple TV, and Amazon Fire TV are all competitors offering different services and streaming options. Most of these are pay-for-service, but some (such as Crackle) are free with ads. Most of them also has a few of their own proprietary content channels, and most of them offer the same Amazon Instant Video service as Fire TV.
