That's what Honda calls a "non-service" part, you can only buy (or could once) the entire pump assembly - #1 on the diagram. So the only source is aftermarket, either find the correct size or get the appropriate diameter cord and glue one up. If it's not damaged I would just put it back in. I don't think it seals any major leak source, only whatever oil leaks past the outer rotor should get to the seal.