if it is a "GARADOR" or Catnic brand, then easy, contact your garage door repairer/suppliers  and get the cable kit for your particular door type, there should be a label on the inside near the internals/internal opener!

the cable will probably come on a cone, wth a pair of short steel rods for winding up and locking of the head spring.... i do take it that it is a "Canopy" garage door..ie, part hangs out when open, and does not have extending inner runners, but small tracks up inside of door frame????

When mine went I got someone in to replace it, it didn't cost much and I kept my fingers

I just did a bit of research, and my spring box above the door is a Mk3c, but the early version with the metal cover.  Apparently it is highly recommended that you DO NOT attempt to change the cables on this version for safety reasons.

They bought out a replacement model with a plastic removeable front which is serviceable, unfortunately mine is not this one.

I have found a reconditioned one on ebay, so someone must have opened up the one with the metal cover, so I think I may go for that.  Just waiting for the seller to confirm the suitable sizes.

Or good solid doors on strap hinges. Bah, all this new technology. You'll be telling us it's not an asbestos garage next.

I think I have the same type of garador door, I have had the cables snap twice and in both instances have watched the repair guy fix them thinking...that looks easy I could do that, but I tell you if it goes wrong you could lose a finger or an eye, to the point I probably wouldnt ever do it myself.
Its not a difficult job to do but you do need to know how to do it.
There might be something on google giving instructions.