There are many sources for sandplay miniatures with a nature or elemental motif: hobby and craft stores, yard sales, and museum gift shops. I have also found wonderful natural items while hiking or walking along the shore. You can take plain objects and transform them for your collection.
Small wooden stars, circles or spheres, found a craft stores, can be painted gold or silver. Small stones can be spray-painted to look like gold nuggets.
Fabric stores have both ribbon and craft items. I have different lengths and shades of green and blue ribbon, tied in a knot at one end, to represent water. Red, yellow, and orange ribbons, tied at one end can represent fire. I buy different widths of ribbon to create these bendable, movable symbols for water and fire. I also look at buttons and other craft items at fabric stores. You might find metal suns, moons, and stars that are used for jewelry making. Also, beads and jewels can be found and are important to include in a sandplay collection. The bright and shiny jewels in my collection have been frequently used as treasure or as a symbol of something highly valued.
Pet stores that sell tropical fish often sell shells, coral, and polished glass pebbles or sea glass. These items can be used by clients to represent the element of water.
Dollhouse miniatures are a great source for birdbaths, ponds, wells, and fountains. You can also find trees, flowers, and plants. Lamps and lanterns for doll houses are good symbols for the element of fire in the form of illumination. With the latest trend of creating miniature fairy gardens, many miniatures for these projects can be found at garden centers and craft and hobby stores.
I also have small ceramic tile pieces in my sandplay collection in a wide variety of sizes and colors. A large blue green tile has been used as a body of water and many small red, yellow, and orange tiles have been used to represent fire.
Go shopping and take your imagination with you!