Have you talked to your surgeon about this? There may be a local bone bank with donor bone available. National bone banks are also a possibility. Studies show good fusion results with donor bone without the complications of pain at the donor site.
Donor bone often comes from people having hip or knee replacements. It can also come from cadavers (bodies preserved after death). The bone that is removed to make room for the implant is ground up and preserved. All bone donors are carefully screened. All donated bone is treated to prevent the transmission of bacteria and viruses. The bone is packaged in sterile conditions so it's safe to use.