The problem is that this is a tagging omitted error. Even relatively common TO errors can be difficult to locate, especially since the passing of Victor Bove a few years ago. I checked my duplicate stash on the off chance that I might have one ... all I could find were a few 1303 & a strip of 3 of 1303a (the untagged precancel). Good luck with your search.
Added: I agree that checking with Craig Selig would be a good place to start, even though his current list doesn't show that any are available.
Added: A longer shot would be tbrstamps on
ebay. He specializes in modern definitives. Once in a great while he even lists an error variety. Several years ago, even though none appeared in his
ebay store, I asked him if he had a 1290b plate block ... he did & he sold it to me at a very reasonable price. You could contact him through his
ebay store.