Apple will still sell the 6S after the introduction of the iPhone 7, but will almost certainly no longer offer the 128gb storage option. If you want a used or refurbished one, you'll have to find a seller other than Apple as they do not sell refurbished iPhones. A large, well established seller on eBay with a high feedback rating above 98% and a long track record of selling iPhones might be a place to start. If you live close enough to inspect the iPhone in person, verify that it is not locked to a carrier that's unavailable in your area nor is it iCloud Activation locked.