If you have original mufflers, then pull the red gasket from the old ones and re use them. Or try your luck with these:
http://www.ebay.com/itm/Honda-C92-CA...182e64&vxp=mtr
If you are using the aftermarket type mufflers then no gasket is available as far as I know. I have used the aftermarket pipes in the past and they don't leak too bad. Perhaps try
http://www.summitracing.com/parts/edl-1077/overview/