What i've done is, as i construct my tombstones, i make a plywood box to attach to the base of the stone. Inside of this box i place a bag of sand.....plastic bag, burlap bag....anything really, and fill it as full as you can. Then attach the box with the sand in it to the base of the tombstone with hot melt glue etc. and finally, paint it in keeping with the tombstone colors so it blends to the rest of the stone. Once done, drop it anywhere on your lawn or Graveyard and it should be pretty stable (the wider the base the better). No driving stakes into hard ground etc. Just pick up and go. Works for me.
P.S. You can also place gravel, rocks, bricks.....anything weighty into the base to keep it stable.