Unfortunately if you wanted a 256GB iPhone, it looks like your only choices are with the iPhone 8, 8 Plus, or the iPhone X. Prior to this, Apple had offered users the option to purchase a 256GB iPhone 7 but according to a report from Mashable, it looks like the 256GB iPhone 7 has since been discontinued. Mashable has since confirmed this by speaking to an Apple customer service rep who told them that they no longer offer that option to customers.
It is unclear why Apple decided to kill off the 256GB iPhone 7 model, but presumably it is to entice customers who are looking for more storage to turn to Apple’s newer iPhone models. As it stands there have been reports that the iPhone 8 and 8 Plus aren’t selling as well as Apple would like, so maybe the company thinks that by getting rid of an older option, customers who need the storage will turn to the new iPhones instead.
Either way if the iPhone 7 is still something you’re interested in, note that it is now only being offered in 32GB and 128GB storage options.