I bought some plastic tags at my local art shop. I don't know who manufactures them as they were loose in a basket, but they come in various shapes. I found that Brilliance, StazOn and fluid chalk inks remained permanent on them if you heat set them. The plastic warps a bit in the process but returns to shape when cool. It was hard to punch the hole though.
Stamps : Stampington piano keys
Inks : DI antique linen, milled lavender; Fluid chalk warm violet; Archival black
Extras : gold paper; DCWV purple card; mixed gold seed and purple bugle beads; plastic tag; metal piano charm; MM and Doodlebug RubOns; threads by Stef Francis; copy of music by Chopin