Where can I find the black crusaders horse barding?
Good places to find parts include:


Thrift stores, car boot sales, jumble sales. Sometimes you get lucky.
eBay is a good general source for parts... as long as you can find what you're looking for.
LEGO's Pick-a-Brick (PAB) service is good for for common parts.
LEGO's replacement parts service is better for less common parts from newer sets. The range of parts is far greater than PAB, but doesn't come close to being complete.
BrickLink, the most comprehensive resource for parts on the internet.


Given that the part you're looking for is older, I suggest that BrickLink might be your best bet.

I couldn't identify the exact part that you wanted, but they do list a good range of horse barding.