5.0-0.8 is an almost universal size for bicycles that are sold in proper bike shops. Just drop into any of them and you can find yourself an assortment of new prop nuts. One item in particular is the outer nut for V brake style brake pads. It is a chromed cone shaped outer with a 5 mm hex in the end. here's a picture of what I'm on about. It's the outer tapered fitting on the posts.

They should have lots of these left over from old pads coming off. A smile and a bit of intrest in their wares and you'll likely get a couple for free if they have any sort of customer relations skills at all.