My girls saw them at the Clearfield County Fair (PA) on August 5th and loved it!
There were about 5 different t-shirt designs for sale, in youth and adult sizes. The two shirts they bought were $30 each (we agreed not to tell their dad the 'real' price!). I don't remember if any of the other designs were cheaper. One of the shirts they bought was pink with 'Big Time Rush' in big letters on the front. The other was a white shirt with the name and a picture of the group on the front and a listing of concert dates on the back.
We also bought a rubber bracelet for $5. These came in black or red, with the group name on it.
We did see the posters mentioned in the previous post, but I don't recall seeing shorts or anything else. It was a small booth, very crowded, and we just wanted to get in, get their t-shirts, and get back out quickly.