Some coworkers and I created a haunted house a few years ago, and we had to do in inexpensively. I came up with this cheap way to create prop bones.
White plastic Tubing (like PVC)
White Balloons 15in (Red Optional)
Grab a balloon, pry it open with your index and middle fingers and shove two wadded up paper towels in it.
Pull the balloon over the plastic tube. Pull it down as far as you can.
Repeat for the other side, you can also use a red balloon on one of the ends.
Next, cut off the rubber end of the balloon while holding the balloon tight around the plastic pipe. Sliding the blade of the scissors around the tube is all that is required to cut the balloon.
While still holding the balloon, use scotch tape to stick the cut balloon to the plastic pipe. Remove the rubber ends so the bone looks flush, without any bump.
If you used a red balloon, touch the blade of the scissors to the red balloon on the pipe. The elasticity of the balloon will open a circle were you've touched it. Touch it multiple times and give the bone a splattered blood look.
There are your bones!! Make many!