You can browse some nice aftermarket sellers. I recommend:
https://www.veblenist.com/
https://delugs.com/
Also, I recommend looking at what Omega offers through its web site here:
https://www.omegawatches.com/en-us/a...straps/catalog
Finally, you can visit or telephone an Omega Boutique, where the staff can help you to select and purchase a strap & buckle directly from them. Visit Omega's web site to search for a Boutique near you, and if there isn't one near you then phone any Boutique you choose.