Umm....any metal plating shop. Look in your phonebook.
Nickel is a miracle metal, 100 times more awesome than chrome to any chemist. Seriously, it's a marvellous metal.
Sadly, chrome is prettier because it's got a slightly blue tint, instead of nickels slightly yellow tint, and and chrome is a little harder. You can really only tell the tint if you see them side by side under good light though. You probably already own things that are nickel, that you think are chrome.
Nickel is ductile though, while chrome is brittle. That can be a good thing if you bang your gun around. The nickel will bend and stretch with the base metal, instead of crack and split.
Also, if you have a chrome plated gun, it's already nickel plated. It generally goes base metal, then copper, then nickel, then chrome.
It also means that if you have a gun that's already nickel plated, it's pretty easy to overplate with chrome if you want to.