Aha! Got one of the Prop Store in London’s items listed before it sold out! So far… You’re looking at Simon’s shirt.
Simon can be seen wearing this costume in scenes on Serenity after Mal (Nathan Fillion) rescues Inara (Morena Baccarin) and afterwards on Haven. The dark blue shirt with an embroidered collar (no size marked) has ‘Simon’ written in the collar in pen.
It’s amazing they kept it this long…
Pricy. Though compared to a lot of the stuff there, it’s a downright steal.