He's a big b***er ain't he!
First source I can think of (for a leg bone) would be a chicken leg or wing.
Take 1 chicken, water, stock, veg, seasoning to taste and 1 nice crusty bread loaf
Make broth and remove bones
Boil bones again to strip all flesh
Wash well
Dry off while eating soup
Soak bones in bleach for at least 30 mins
Dry off bone and let dry in the sun or on top of a radiator
Stain / varnish bone if you wish
If you are a vegetarian or vegan the only other option would be to get some toy dino bones kits
http://www.ebay.com/bhp/dinosaur-skeleton and see if any of these fit... I have some 15mm 'sized' ones but nothing the size you need...