If all you want is a sump washer then pop into a local motorcycle shop and buy one. If you are also buying other stuff for the service like an oil filter, then try wemoto (won't be worth the postage just for a sump washer).
I'd replace it every time, I mean what's the point not doing so for 25p.
Some people will reuse them, I've done so in the past, but why take the chance its buggered and you only find out after you've taken the sump plug out.