Human Anatomy and Physiology - The Muscoskeletal System
For bone grafts, they usually use parts of the bones, so their removal should not result in problems with the "body weight maintenance" or "posture."

Parts of the fibula (calf bone) can be used, for example, as a graft to repair avascular necrosis of the femoral neck or congenital pseudoarthrosis of the tibia (shinbone) (International Orthopaedics).

For spinal fusion (joining the vertebra in the spine), they typically use the bone from the iliac crest (the top part of the pelvis) (OrthoInfo).

Other bones that can be used as grafts:


Parts of the ribs and certain skull bones for reconstruction of the facial bones
Parts of the mandible for tooth implants
Upper part of the tibia for various purposes 
Distal part of the ulna for correction of the wrist in rheumatoid arthritis