Ah. True Blood. Such an endless source of hot people, ridiculous acting and plot lines, and hilarious accents. I love you. The easiest person to dress up as is Sookie (although Pam would also be a hilarious option). She doesn't really have a lot of costume changes. It's either the Merlotte's uniform, jean shorts and a crew neck, or a retro, floral sundress. The jean shorts option is boring, so I ignored it.
Sundress, and Merlotte's uniform. The keys to the rest of the look are minimal makeup, long blonde hair, and possibly fang marks on your neck (easily achieved with red eyeshadow and black eyeliner).
Sookie Costume Set, $44.98 at hbo.com, J. Crew 3" Chino Shorts, $24.50 down from $39.50 at jcrew.com: You can buy the legit Merlotte's outfit at the HBO website. $45 isn't cheap, but I would totally wear this more than once. Probably the shirt during daily life. I'm really cool. So you do the shirt and the apron, and then she usually does a pair of basic black shorts. These J. Crew shorts are perfect, and they're totally on sale right now. Get on that. Add a pair of white Keds and you're good to go.
Modcloth Floral Fiesta Dress, $63.99 at modcloth.com: On the other extreme you have the retro-girly sundress. She really only does the extremes, nothing in the middle. Whatever. I like this Modcloth version because you could wear it after Halloween. The cut is flattering, it says Southern charm but it's not so kitschy that it isn't wearable, and that ruffled skirt is just so flouncy!
Images from tvfanatic.com, truebloodguide.com, hbo.com, jcrew.com, modcloth.com.