Hi Alex,
Unfortunately IWC has a very (very) small selection of straps they make, and
since it's a proprietary connection, there really aren't any options on the
secondary market.
I haven't found any myself, but after I bought a new IWC strap:
- I took my old strap to a strap maker
- he stripped the original material off the rubber
- I selected a leather and color and he put it on the old strap
or you probably could send your old strap to someone and ask them to dismantle
the old strap in order to reuse the connection section in a new strap. This
way, you should be able to customize your own strap 100% from start to finish
(material, color, length, etc). The way I did it I have to use the old rubber
strap section.
Hope this helps and have fun customizing your strap!