Goto any Sports Authority or Modells for plain black sweatpants. The ones with elastic waist & cuffs. Those are fine for NPC gear. As an NPC, you'll be running around, getting sweaty, dirty, muddy, and overall just gross most fo the time. Just brings clothes you'll be comfy in while being very active in the summer heat. That's my advice. Any special costume stuff will be handled by the Marshals on duty. See if you can get a belt pouch or fanny pack. Something to hold cards, fake gems, treasure, & other stuff as you NPC.
Socks... lots of socks... it gets wet and muddy at the event. Maybe a pair of back-up boots or sandals. If the weather does turn hot, & we can get a lifeguard present, you may wanna try for a dip in the lake. Bring swimming trunks, a towel, & maybe some bathing supplies.
All in all, read the New Player's guide/write up as it has a lot of great stuff. If you're only NPCing, it does make your costume requirements a lot simpler and no need to bring weapon props.
Last thing... welcome aboard. Have fun. Don't be afraid to mess up. Listen to the Marshals. Hope you have a great time and we do our best to have you join us.
IG: Tyran Radley
Vassal of Alisandria
OOG: Paul Y.