This allows you the best possible prices, and a neat assortment of things from which to choose. Or, you may choose, instead, to build everything from scratch. Yard decorations are not hard to make. They are usually unexpected and can cause quite a fright for your visitors, especially if they are partially concealed behind something your guests must pass around. You might even consider adding some movement and sounds to your props, to further the scare factor enjoyed.
Tombstones are also a great idea to use.
You can make them with relative ease, using Styrofoam sheets, cardboard, etc. And, with these, you can build a grave quite easily. If you'd rather not do any digging, simply use sand, potting soil or other dirt, to pile into a grave-shape, adding more realism to your gravesite. If you choose a buried grave, or one actually above ground, you want to pack it down carefully, especially if you think visitors to your yard might happen to walk across them. You don't want any guests to accidentally fall.