Look on ifixit.com and macsales.com . Or Google “MacBook air SSD upgrade”. Lots of options. Best kits include an external case for the old SSD drive so you can use it as an external drive (though it sounds like you won’t need this).
You will need to know the specific model of MacBook Air when ordering. Look on the bottom case to read the model number. If it’s bootable from an external drive, you can check “About This Mac” or System Information in the Apple menu for more info about your Mac’s model year.