Great taste in pairings, the SL63 with the matching IW500507 is one of those AMG era IWC collaborations that has genuinely held up. Only 1000 pieces made, and most of them ended up on the wrists of the original SL63 customers, which is exactly why finding one on the open market is so frustrating now.
A few honest pointers for the hunt.
Chrono24 will throw up two or three listings a year, but the asking prices have been climbing steadily, anywhere from 12k to 18k depending on condition and whether the original box, papers, and the AMG presentation case are all present. Without the full set the value drops sharply, so be patient and wait for a complete example, it makes a real difference on resale later too.
Watchuseek's IWC subforum and the dedicated AMG owner communities (especially the Mercedes-Benz Club forums and a couple of the larger AMG private Whatsapp groups) are where these tend to change hands quietly before they ever reach a public listing. Worth posting a WTB notice there as well.
The other route worth trying is reaching out to IWC boutiques directly, particularly the larger ones in Germany and Switzerland. They sometimes have customers looking to part with limited editions and act as informal go-betweens. Not official policy, but it happens.
And good news for you, overtheedge above is offering one with the matching car still in the family. That kind of provenance, AMG sourced original owner with the watch and car still together, is about as good as it gets for this reference. Definitely worth following up on.
Good luck, post pics when you land it.