Coventry uses quite a few suppliers, but you are correct, they get the forged steel link from the Spares Co. It is for this reason I am not sure you can find another source, so I would stick with Coventry.
Coventry used to supply Buchanan's spokes. If you want stock sizes quickly they may have them on the shelf. If you want something custom, like spoke or nipple sizing I would go to Buchanan directly. If you are lacing an existing rim I would always ask what size nipples are supplied because the rims may be drilled for a different size.
David