Previously I'd cannibalize I/6 equipment, but this is now becoming a rich mans game with 1/6 accessories costing a lot more than they used to.
Phil Woodard put me onto an ebay supplier. not sure if this is the exact one, they have a limited range of sizes and colours. I would say the material is slightly shiny that could be changed with paints or dyes probably. I wouldn't try to cut to a thinner width because I think the material will fray.
Smallest width is 6mm sadly but still very useful with Armorpax brass fittings. 6mm x 1m £2.80 inc p+p
Put this in the ebay search to get the listing:
16 TYPE Grosgrain Ribbon 6mm to 40mm Hair Bow Seam Tape Trim.
Colours shown are dark green, brown and dark brown.