There are Bookstore/Gift Shops at Kilauea Visitor Center and at the Jaggar Museum which are operated by the non-profit Hawaii Natural History Association.
What to buy: We got our National Park passports stamped for free at the Visitors Center (photo 2). We could have bought hoodies, or books explaining petrogyphs or Eruption Update DVD
Beside the videos, coffee table photo books, and other gifts typical of an national park gift shop, we found these nice unmounted block prints that were individually signed and numbered. Since they were less than $10-, we found that to be an excellent deal. The subject images we bought are Hawaiian culture activity themes. The store has many other things of interest too.
What to buy: As said, we found the block prints a good buy in terms of original art. The images here are deliberately bad, but they provide a good idea of the art.
What to pay: Ranges depending upon size. We bought smaller ones for less than $10 each and then good four of them.