Remington factory extensions were available in polished blue (as well as matte and parkerized); two and three shot.
You can look at online retailers but I haven't seen those commonly available in a couple years. Best bet is probably eBay.
More involved option, if it means that much to you, would be to find a currently available extension from any maker and confirm it's carbon steel, and pay to have someone polish and blue it to match you Wingmaster. Probably more trouble than it's worth, but it's an option if you can't find a new old stock or used factory extension.