Used hardtops for a Mazda Miata are available online at TreasureCoastMiata.com, GoMiata.com and eBay.com, as of April 2015. The hardtops are for various model years, including for the 1990s and newer models.
Treasure Coast Miata offers a hardtop made of Fiberglas that fits the dimensions of the original part. The top needs to be painted, but it comes with a coat. The site calls it a racing part, because it weighs about 35 pounds less than an OEM hardtop.
GoMiata.com's removable hardtop fits the 1990 to 2005 MX-5 trim. Users have several customization options: rear glass type, window size, top carrier and interior color.