Try ebay where I have purchased from before, made sure its a Bluetooth and not just an spen myself search website for a spen got one with half the function found out there are different types of spen, so search for "Bluetooth spen " for your device ok
Definitely eBay is the cheapest option. I mean, I've bought one from Best Buy for like $10 or something like that but I feel like I got one from eBay for $5 shipping included. I think they take the pen from a broken phone and sell it as is. I'm not sure. It seemed brand new. Not much of an issue. But Best Buy does have them if you're in a pinch. Make sure that you get the one for your model though. Most of them are the same but I wouldn't want you to not be able to put it inside the phone.