If your having trouble finding a 15T sprocket then a tip is that they are the same fitment as the Honda XR250 trailbike.
Here's a link to the sprocket on the Motozone website, I've used these guys numerous times over the years:
MTX OE Steel sprockets are strong and durable, heat-treated and hand-finished for longevity. Front sprockets are constructed from SCM420 chromoly steel alloy where rear sprockets are constructed from C45 high carbon steel with a Zinc electroplated finish. Designed as an OEM replacement to exceed...
Regarding aftermarket slip ons: Yeah unfortunately in NZ there is not a lot of scope inbetween the $899 Yoshimura and the cheap Chinese pieces of junk for $150 on ebay. Dont be tempted to go down the shorty route unless you want something obnoxiously loud that makes it sound like a cross between a lawn mower and a dirtbike.
Delkevic are a good middle ground option but bizarrely they dont make one for the 300R, only the 250R which annoyingly has a slightly different mount point and header length. I managed to make it work though by chopping a couple of inches off the 300R header but it never sat quite right. It needed a custom made link pipe between the header and the slip on with just the right bend. Not an impossibility but not sure how much a muffler shop would charge for such a service.
Here's the link to the Aussie seller anyways:
DetailsThe attention to detail ensures fit and performance giving you a product that has proven to stand the test of time at an unbelievable price.All necessary copper and fibre gaskets are supplied with the exhaust, where applicable, completely FREE of charge, saving you $$$$!We manufacture all our