Looks like it takes a standard low shank. There's several styles out there. These two are metal.
#4021-L FOOT Embroidery/Darning Low Shank Giant 12mm Metal -- $9.90
#7773 NN(Nifty Notions) Quilt/Darning Low Shank 12mm Foot Metal -- $9.40
Nifty Notions