As
@Kad says ebay is your go to for all sorts of hard wear, also webbing, pleather or leather for straps. I love the look of leather straps, they really add a classy touch. I bought some recently but it will take some time to get a nice feel, at the moment it’s very stiff and new looking.
Another option often suggested is to reuse something like an old belt or straps off something else. That works for something like a tote bag but would be very difficult for the length of strapping you would need for the style you want.
I would go for webbing rather than PUL straps, the artificial stuff soon looks tatty.
As far as I can see you are only looking for buckles and triglides and some form of strapping. Why not give it a go with some webbing and the right hardwear, if your happy and comfortable with that you could upgrade to leather once you know exactly what length you need.