You can watch Season 2 on the Disney XD website though you do have to enter information about your cable account to access it. I'm guessing this option would be good for those of you who have already watched the episodes but forgot to record it so you could watch it a second and third time. You know, like normal people.
iTunes has both seasons available in HD. The Siege of Lothal is $7.99 and you can buy the season pass for the second season for $28.99 (meaning you'll have access to watch the entire season as the episodes air). You're also able to buy each episode separately at $1.99 for Standard Defintion or $2.99 for High Definition. This is a good option as long as you're ok with only being about to watch it through iTunes on your computer.
Google Play allows you to purchase the episodes separately as they've aired. They are $1.99 for Standard Definition or $2.99 for High Definition. I figure most people enjoy HD but it's nice that they give you the option. You can stream the episode from the web using Google Chrome or Chromecast and you can also watch it on most Android devices. Plus, if you have an HDMI cord, you can connect it to your device and watch it on your TV! We decided on this one since we had multiple different ways of watching it.
Thanks to The Wookie Gunner for letting me know that if you're a Sling TV member ($20 a month) you can watch Rebels through Disney XD for $5 extra. Sling TV allows you to watch live TV customized to your interests with special packages that you can add on for under $10 a month including sports and family channels.
I hope this helps you in your quest to watch one of the best shows ever! And do let me know if you discover any other places to watch Rebels so that I can update the list.