You can purchase the appropriate size secondary, 63mm, for the 10' mirror from either Bintel or Andrews Com. for about $50 . Not sure what the surface accuracy of the Chinese secondaries on Ebay or AliExpress is like. They may come from the same supplier to G.S. or Skywatcher and be ok but the price difference is bugger all in the end so buying local is safer.
Have you tried gently washing the secondary, in the recommended methods such as
http://www.iceinspace.com.au/63-345-0-0-1-0.html and many other astro websites have similar instructions a well. The coatings on secondaries usually last longer than on primaries as they are facing down and not subject to the same amount of exposure to the elements.