You want human hair.
For that Kelly curly look she had last year I recommend hair from two places. Either naturegirl curly www.naturegirl.net/curly2.html You'll want the wefted in 14" and you'll need about 14 wefts which run 19.50 a weft so about $300.
The second place is extensions plus.
http://www.extensions-plus.com/zzw.html, you'll want the natural curly. You'll want about 3 packs so about $300 You can click on this link to see a pic of the extensions plus hair that someone bought.
http://forum.blackhairmedia.com/forum_posts.asp?TID=13908&PN=2
Both of these are a little pricey but will give you the look you want. If you want a cheaper alternative, you could try bohyme hair. I, personally have never worn the Boyhme curlier or wavy textures but I like their straight.
http://www.superhairfactory.com/sho...Path=2&zenid=07bd4f6e49fac5f5f05e945b91cead81
If you get the Naturegirl, the wefts are hand tied and can't be cut so your stylest has to know how to work with those types of wefts. you can cut them and tie them off but you lose a lot of hair.
Here's another brand that's okay. It's not as good as NG or ExtensionPlus but it could work if you want another cheaper alternative. It's from adorable
http://www.adorablehairdo.com/Overview.html But my recommendation would be Naturegirl though.