I stumbled upon a company that makes ordering stones for unmarked graves simple and very affordable. I haven't used their service yet but it sounds very interesting. Mark Every Grave allows you to order small granite headstones (6" x 9") starting at $25.99 for 6" x 9" x 1/2" stones up to 24" x 12" x 1/2" for $250. You can select an image to have engraved in addition to names and dates for a small amount more. There is also an option for a custom image. Their web site notes that shipping is free.
Assuming the cemetery allows it, you could have all unmarked graves of your extended family marked with these for not too much money. You could use these as temporary markers until the funds are available for a more expensive stone. I've also seen where someone will order this as a temporary stone, then have it affixed to a larger stone at a later date.
My great-great-grandparents, Thomas and Catherine O'Hara Doran are in unmarked graves in Warsaw, Hancock County, Illinois. It's tempting to find the exact location of their graves if possible and have these stones set for them.
Again, I haven't used their service yet so this isn't an endorsement of the company but it is an option if you have some relatives you're considering getting temporary markers for.