The weighted plates you buy at sporting goods stores are likely made of steel or iron, and are sometimes coated in rubber. You can make cheaper plates yourself out of sealed concrete. Constructing a mold that produces the appropriate weight for each plate takes time, but the overall process is something anyone can do with basic tools and hardware store concrete.
Mix the concrete per the instructions on the back of the bag or box. Pour the concrete mix into a 5-gallon plastic bucket, then add the water. Stir vigorously using a stirring stick.
Build a square wood mold in which you will pour the concrete. Cut 2-by-4-inch boards to the appropriate size for your mold, which is dependent on how heavy of a weight you require. Large molds will produce heavy plates, while small molds will produce lighter weights. Screw the pieces together using wood screws and a drill. Cut a square of plywood large enough to cover the back of the mold, then screw it into place.
Measure your barbel's bar diameter using a tape measure. Purchase a pipe with the same diameter. This will be the mold for the plate's hole.
Pour the concrete into the square mold. Insert the pipe piece into the center of the concrete. Allow the mix to dry.
Flip the mold upside down and pound the bottom with a hammer until the concrete comes out. Smack the center pipe mold if necessary, then pull it free.
Weigh the concrete plate on a scale and write its weight on the side of the plate and the mold using a marker. Make a smaller mold for a lighter weight, and a larger mold for a heavier weight.