The flush tube is unlikely to leak until you flush the toilet and if that's the case try tighten by hand a little more that maybe the problem. If still leaks but all the time then it is most likely the syphon nut on the base of the cistern hold the syphon head with one hand and give an extra turn a bit on the nut. Do the flush nut back up again holding the syphon head tighten.
Try it. I've never looked for these washers, tapered on the flush pipe and flat on the syphone inside. I've had same and carried out above and it stopped. I've never had to renew a washer on a syphone and as Pecavvi says you might not be able to buy these seperate but try DIY store or Plumbing shop you might be lucky if above doesn't work.
Charlie
August 2010