I assume you're already running a 16Tooth countershaft sproket. With regards to the 41T rear, any KTM dealer will be able to get you the KTM Stealth Rear Sprocket, they are a little spendy though at around $115-120US.
There also appears to be another company that makes 41Toothers for our bikes, that company is . I've used them on my Ninja and they seem to be really durable (steel) and properly priced. I'm betting that you should be able to pick those up from any Parts Express dealer. People usually don't have those in stock in a brick-and-mortar shop, which you should be supporting, so it may take a couple of days for you to get it when you order them.