You can make your own casting sand for almost nothing. Then use a
plaster, wax, or wooden master to cast as many pieces as you want.
Sieve some regular plaster sand through a fine screen and mix it with a
little flour, water and glycerin to make a packable mixture. for an
open pour, just one mold is used. Place the master, face up on a sturdy
table and ram the sand into a box form around the master. When the sand
is solid to the top of the mold, turn it over and carefully lift the
master out.
Heat the mold in a hot oven just before pouring to drive off any water.
The sand can be re-used and reconditioned. Three dimensional castings
are made with a split molding box called a cope and drag.
Get a book on casting to learn the details.