Aluminum cans are durable, as are baking sheets, for making plant identification markers. Aluminum cans can be cut apart by scissors or metal cutters. Thin aluminum baking sheets can also be cut apart into strips the same way as aluminum cans. Both can be engraved on or sticky labels attached. Permanent markers should be used to write on the labels and a lacquer can be applied to seal and protect the writing.
Flat stones and stepping stones can be used to write on with permanent markers or paint. Stepping stones, such as brick and ceramic round and square tiles, can be used. These also blend into the garden and do not stand out. A sealant can be applied to these as well to protect the writing.
Scrap wood is suitable for plant identification markers. They are easy to write on or sticky labels can be applied. Wooden stakes are also available for purchase in multiple sizes. If making them you can create the size that right for each plant in your garden. A wood lacquer can be applied to seal and protect the wood as well as the writing.