Best/easiest upgrade is any SSUBX drive. SSUAX almost as good. Search for those terms on eBay. Note 1TB won't fit as too wide. Also don't need one with heatsink.
To be clear: Apple laptops **cannot use** SSUBX-with-heatsink. Products with model number ending in /0A[23456] definitely do not have heatsink. Products with model number ending in /0A1 either definitely do have heatsink or possibly do have heatsink. Example model number: MZ-JPV2560/0A4.
I recommend you take a look thru this thread. I posted how I installed in my 2015 MBA (a 512GB NVMe M.2 drive) there...been using this configuration since March2018 without any issues so far...