Call a full-line craft/art supply store. There is some kind of liquid stuff that you can pour into a mold to make your own paperweight...I'm not really sure what this is called, but I'll try to find out and get back to you.
I have never done this, but my husband's family used to collect shells at their beach house and let the kids make paperweights to keep them busy. It was still available as of two years ago. My brother-in-law was in charge of purchasing the products, so if I can track him down I'll find out the particulars.
By the way, this method is obviously not archival....
Good Luck!
Leslie