You can either buy direct through her websitse - http://www.serenitydesigns.org.uk or her ebay store - http://stores.ebay.co.uk/serenitycrossstitchdesigns depending on which you prefer.
Her kits are fabulous, especially if you're a bit of a geek, but even if you're not I recommend going and having a look. She's got kits of CSI, Twilight, Buffy, Merlin (which I want!) Game of Thrones, Serenity, Lord of The Rings and so many other things - I was basically in geek heaven, there are so many that I wanted, it took me absolutely ages to narrow it down to just 1, and even now I'm wavering towards a couple of her others. She also does threads packs for HAEDs and also some HAED charts (authorised retailer, don't worry!).
I ordered through ebay, because I hadn't discovered her own site at the time and received excellent service. The kit was dispatched really quickly and I got message telling me that it had been dispatched, and it arrived I think 2 days later. The kit itself is great, comes with a front cover picture, a chart (obviously) over 6 pages in this case, all your threads pre-sorted and labelled (I photographed it the wrong way up, so you can't see the labelling, but it's all really clear!), a key and a needle. The chart's really clear, and I'm itching to start it (but seeing as I've got 7 on the go at the moment, I'm trying to resist for a while).
Service was great, excellent communication and arrived in great condition and good time. On an additional note on communication, Anna's also got a facebook page for Serenity Designs, making her even easier to contact, and it also means that you can see other progress pictures as well.
I love Anna's stuff so I hope a few people pop over and have a look, and hopefully are maybe even tempted by them! If you do make sure you post on your blog so I can see what pretties you've bought and I can stitch the others that I love vicariously through all of you!