Most of the time the black finish on an aluminum part is just anodizing. Sometimes you'll have a powder coated part but generally thats not the case. To remove it you could try copper wool or fine steel wool and/or a brass brush on a drill. Medium grit sand paper will work as well. You want to make sure what you're using won't damage the bar structurally and only removes the relatively thin anodizing.

The only bars I could find in chrome/silver after doing a quick search are these:

http://www.jensonusa.com/Handlebars/...-Evo-Riser-Bar

http://www.amazon.com/Origin8-Pro-To...3&sr=1-3-fkmr1