The print graphic designer´s heaven. All sorts, sizes and weights of paper, shaped in forms i have not seen before (jup you don´t get that in Austria). Packaging material, adhesive letters and everything else you just want to have. Find all sorts of pens and versions of Moleskines. If you like childrens books and/or illustration, also check out the great store on the other side of the street (Cupcake Café).
I think it takes place every Saturday morning to afternoon, vintage shopping while having a great view at the Manhattan skyline from the East River State park. You will find lots of vintage shops and warehouses around this area of Williamsburg.
This is a pro tip, but also a summer tip, take the ferry from Lower Manhattan to Rockaway and spent the day. The beach is great and the area is impossible to gasp that it's 30 min from Manhattan (without traffic) - Eat as many tacos you can, cause it's the best in the state of New York!