For spousal sponsorship there is NO requirement for reference letters. But if you have them, they may just strenghten your case, that is all, so do not sweat if you do not have them. I did not have and I do not have any, I provided only what was required per the guide, and for additional proof, we have many pictures together, tons of transcripts from our QQ and YM chats, monthly telephne bills, as we talk each day once or twice, plane tickets, hotel receipts showing both our names, some mail correspondence and that was all.
Best of luck.