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You can find these shells labeled as "lion's paw shells" in an arts and crafts store. You can also find similar shells made out of plastic in a costume shop or a party supply store. Make sure that the shells match up in size and shape. For a child's costume, you can cut two large shell shapes out of felt. You can match the color to your tail, or you can use a contrasting color. For example, if you made a green tail, you could paint the shells purple. Sparkly, metallic paint would work the best, but you can use flat paint as well.  Use acrylic paint or spray paint. Let the shells dry completely, then add a second coat of paint, if needed. Mix extra-fine scrapbooking glitter into glossy decoupage glue (ie: Mod Podge). Paint the glue onto the shells, then let it dry. Apply a plain coat of glue on top to seal the glitter in. If you want extra sparkly shells:  Paint the shells with decoupage. Shake the glitter on top, then tap the excess glitter off. Let the decoupage dry. Apply another coat of decoupage on top. If you still want some bling, but don't want to use all-over glitter, you can draw designs onto the shells using glitter glue instead. If you don't have any glitter glue, you can use sparkly puffy paint/dimensional fabric paint instead. You can also draw designs using regular glue, then shake glitter into the glue while it's still wet. You will be building your bra on top of this. A child or a teen could use a skin-toned tube top or tank top. And adult could use a skin-toned bra, preferably strapless. Alternatively, you can match the color of the top or bra to the color of your shells. Coat the inside of the shells with fabric glue or industrial-strength glue (ie: E6000). Place the shells onto the bra. You can have the bottom edges point towards the center, or you can have them point downward. If you are using a tube top or a tank top, you will need to position the shells where the breast would be. Industrial-strength glue would be the best for this, but you can try using fabric glue as well. Hot glue will work, but it may not last as long. Here are some ideas to get you started:  Apply tiny rhinestones to the ridges on the shells. Glue a mini starfish to the middle of the bra where the bow would be. Add a sequin or rhinestone trim to the edges of the shell. Decorate the bra band with rhinestones or fake pearls.
Choose two large shells to use as your mermaid bra. Paint the shells, if desired. Consider adding some sparkle with glitter. Add designs with glitter glue if you want something more subtle. Find something fitted to serve as the base. Glue the shells to the base. Decorate the top further with gems, pearls, and smaller shells.