Insulating HVAC junction in unconditioned attic
Are you certain it's not insulated on the inside? The majority of metal duct is made with internal insulation.
Regardless of that fact, yes, you can buy duct wrap. It's basically fiberglass insulation with a reinforced metallic covering. It's usually silver in color but it is also sometimes white. It is attached using brand specific tape that is about 3" wide, or you can buy aluminum tape in 2" or 3" widths at a big box store for about $7.00 a roll.
"The physician can bury his mistakes, but the architect can only advise his clients to plant vines." -- Frank Lloyd Wright