You can use scrap pieces of 2-inch-by-4-inch pieces of wood cut to the highest you want your bed raised. Assemble four pieces of equal height into a square and glue and nail them into place. After the glue is dry, trace out a square to fit into the opening of the block, cut and glue the square into place. Once you have cut and assembled four blocks, lift you bed onto the risers. Make sure to check the blocks periodically for cracks and uneven wear.
You can have cinder blocks cut in half at your home improvement store to use as risers. Fit a half cinder block under each leg of the bed frame to make the risers. Check the cinder blocks periodically.
Bricks that have been stacked on top of each other make a for an adjustable bed riser. Stack as many bricks you need for the height you want under each bed frame leg. Make sure to inspect the bricks for cracks before you use them.
Instead of making blocks with wood, get some 4-inch-by-4-inch beams from your home improvement store. The home improvement store should be able to cut the beams to the height you want. For extra stability, use a hole-cutting drill bit that is slightly larger than the legs of your bed frame, drill a 2-inch-deep hole in the top of each beam. Slip a beam under each of the bed frame legs and check often for damage.