It appears that that specific copy of this shirt was just sold in the last several days, so could it be that the shooter was the purchaser? If so, might be an important lead to his (her?) identity.
After a google image search, that shirt is sold in a billion places. Could have picked it up in a yard sale in Arkansas, would be impossible to track the sale in this case, and not worth the wild goose chase there.
You’re assuming that the shirt sold in that listing is the one the shooter wore. They could have gotten it just about anywhere.