The NY Lange boutique used to offer customization for straps through JR if they didn’t have a color a customer wanted. I haven’t been there in a while so maybe things have changed as the brand has become ‘popular’. JR can also give you the Lange strap boxed-end finish (not sure what else to call it!).
Its always best to see the colors in person but there is a beautiful darkish blue that JR carry that is pretty close (to my eyes) to the 25th anniversary straps. The strap no. Is 7652 - full credit goes to our mod CR who mentioned it in one of his posts and its stunning in person. If you do that with a grey or silver stitching you are pretty close. Of course, nothing beats sifting through the leather swatches in person and the JR store in New York is extremely helpful and are willing to spend a lot of time with you if you call ahead (or walk in at the right time).
The other alternative that I usually go for is Kaufmann. Wempe in NY carries their catalog and can order from a variety of options. The downside is that they don’t have swatches on hand so you will have to go with the pictures but the end product to my eyes is also really close to the picture. For what its worth, Kaufmann used to make Lange’s straps in the past and can make the Lange boxed-end finish if asked. I personally find their quality to be as good as JR and CF and pricing is around the same or a little less depending on configuration.
Hope this helps and happy Thanksgiving!
All the best,
Walid