As a non-uniformed group of resistance fighters, there are no official uniforms. This allows you great flexibility in choosing what you want to wear in order to fight the machines. This includes base clothing, ammunition carrying rigs, drop leg holsters/pouches and anything else that would be useful in a tactical environment. Based off multiple timelines, here are some of the more popular options for Resistance dress.
The base of the Terminator Future War soldiers is a grey jumpsuit. Feel free to explore around to find a grey jumpsuit we found a Dickie's one that could be modified to better look like the one from the film. If you want to really look screen accurate you may want consider purchasing a quilt and adding the pads pictures on the shoulders by sewing them on.
For the red armband, the most authentic thing to do is just grab a red shirt and cut off a sleeve or the bottom. This matches well with what the resistance would actually be using since in the post-Judgment Day world they are using whatever they can find. If you don't want to cut an existing shirt, just pick up a cheap red shirt and cut off some of it.