I fill a whopping great tupperware box with tap water and put it in the freezer overnight. Next day I turn out the ice brick onto a clean towel on the floor, fold over so it's fully wrapped, then beat hell out of it with a hammer or rolling pin.
You can stop at whatever stage of lump size you desire. Shovel the lumps into a freezer bag or two and throw back into the freezer for keeping.
This method is cheap, easy, fun, therapeutic.