Do the earth a favor and throughout the year snag...
STYROFOAM PACKAGING - our office was upgrading all computers and the inserts protecting electrical components refused to end up in a landfill. The styrofoam packaging is ideal when repurposed into a green graveyard
EXPIRED CAMPAIGN SIGN SUPPORTS - do the neighborhood a favor and remove stray election signs from public locations the day AFTER the election. These are great for securing the outdoor decorations
"OOPS" PAINT - Home improvement centers will often sell mismatched paint (cheap!) and are willing to tint it grey at no extra charge. "Free boxes" at garage sales are another source. These will come in handy when trying to give the appearance of aged marble/concrete.
PLASTIC FLOWERS - what thrift store doesn't have an overabundance of plastic flowers waiting to make another appearance...even if it is at a GREEN graveside?
CHEESY KNICK-KNACKS - prayng angels, infant cherubs, doll heads...can all be painted black/grey and secured to tombstones for an added touch.
While most of the materials can be acquired on the cheap (practically free!), investing in a few tubes of Liquid Nails will be your only major expense when securing layers of found items together.