Hi mate sorry to bump an old thread but just incase you didn't work it out, if you take the pad out and check it against a chart of common brake pads, it will probably match up with something already available. I've no idea what calipers are on your Onza, but its either going to be something like a Tektro or Promax etc with Onza written on them, or maybe an original Onza brake but either case will probably take a Shimano Deore pad. There are loads of designs but thats the most common, then loads more companies make the pads, just see what Tarty or your local shop have.
If you send a picture here, me or someone else can help match it up 🙂
Adam