I did not know who Sweetie Belle was before a week ago and I come to find out she is a one of the newer ponies on My Little Pony which I guess is making her a Hot Holiday Gift this year as everyone tries to find her toys. We were able to find the vinyl version of her from Funko pretty easy, and I would consider that an option if you cannot find the plush or in addition to the plush, but searching for the plush was a bit of a struggle.
Sweetie Belle is a school-age unicorn pony and Rarity’s younger sister. She makes her first major appearance in the episode Call of the Cutie. Sweetie Belle and her friends, Apple Bloom and Scootaloo, form the Cutie Mark Crusaders, a club/”secret society” devoted to helping ponies earn their cutie marks. (From http://mlp.wikia.com/wiki/Sweetie_Belle)
It is currently (10/6/2016) sold out at Walmart online (in store only) for the My Little Pony 5″Sweetie Belle and the My Little Pony 12 Inch Plush – Sweetie Belle is out of stock at Toys R Us and the 10 inch one is out of stock on Kuurupop.com. You can find them on Amazon and Ebay, but if you just keep your eyes open they may pop up in stores and online again. If you really want a Sweetie Belle Plush you are going to have to do some work. There are the ones that are being shipped over from China on Amazon and I have already received mine in great condition so that is the path that I went.