Ive got one and it is super accurate. However the adjust up and down wheel on the rear sight is missing. Any idea where I might get one?

The older Winchester 353 and 363 pistols were made by Diana (also imported by RWS). Pyramyd Air has replacement rear sights for the Diana's. Their inexpensive, replace the whole sight or take the part you need for yours. the elevation screw looks the same on all my Diana/RWS pistols and rifles. Also try Umarex current importer of current Diana products