hardware stores, leather/handicrafting stores, specialized button/ haberdashery stores, or perhaps some cobblers could provide the customised service.
you want to look for nylon webbing.
they normally come in rolls but most are sold by length.. or you could scrap the material from a discarded bag sling or straps. same thing with leather and you might also require some leather dye...
as for buttons, be prepared to invest
in the required "snap button
fastener kit"
be certain that the buttons aren't steel
(testing with a magnet would be handy)
but brass.
that is not unless you require using
the bright silver chromed ones ,
which can and will develop rust
under the right conditions ;-)