Hey there Eric,
So iFixit does not carry the back glass yet. If you are looking for an official part, Apple will sell you the part through their [link|https://selfservicerepair.com/en-US/order|Official Self Service Repair Store]. The only caveat to this is that it will be more expensive to buy it directly from Apple. Otherwise, you can try to get the part from a damaged donor phone off of eBay or other resellers, but their is the risk that the part won't be an official OEM part which might cause software issues.