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As far as i know you cant buy oem spyderco parts like that.
The standard backspacer for the FRN-handled Endura4 can be found in the 'Lightweight Parts Kit' for that knife. That kit also includes replacement 'nested' liners (skeletonized), but not full liners.
http://www2.knifecenter.com/item/SP10KIT/Spyderco-C10KIT-Endura-4-Lightweight-Parts-Kit
As mentioned, I don't think full liners (or metal backspacers) can simply be bought outright. Probably would have to have them made for you, unless you can find & cannibalize one of the G10-handled Enduras, which do have the full liners. In that event, I'd think it'd be simpler to buy one of those knives, then replace or customize the scales and whatever else to your taste (assuming this is the goal, anyway).
David