One of the buckles my (used) 900LT's bags was replaced with a blue buckle when I bought it. Somebody used a piece of black leather to sew the buckle to the existing saddlebag strap, and it matches the other except for color.
You may be able to find a matching buckle at a camping/outdoor store. These types of buckles are frequently used with nylon webbing for compression straps, climbing gear, etc. Buckles should be relatively cheap, I'd recommend you try to buy locally since shipping cost would probably be more than the buckle itself.
Not saying these specifically would work, but take a look at something like
or maybe www.strapworks.com