The question is all in the title. I'm talking about this: the second knight from the left entrance to the castle.
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.