The cheaper way in? The Older geneve

Strap width should be 18mm at the lugs tapering to 16mm at the buckle. For a dress watch, alligator, crocodile, teju lizard and shell cordovan are all good. There are other exotic skins, like ostrich, that can work too. There are many excellent aftermarket strap manufacturers, including Hirsch, Artisan, Rios1931 and lots of others. There are plenty of existing threads on OF about straps that you can view. As for strap color, brown or black are traditional with a gold case, but choose what you like. It's your watch. eBay is a good source for vintage buckles, but be careful because fakes abound. There's also a very good thread on OF about Omega buckles that you can search for and read. When ordering a buckle, make sure that you order a buckle based on the width of the strap at the buckle end, not the lug end. So for an 18/16 strap, you want a 16mm buckle.

Something like this maybe

There are very specific markings that identify original buckle you can post photo of something you’re considering here first. If you don’t want to spend for original you can find AM buckles for much less.