Where can I buy the apc nos to speed it up?

To speed up your APCs there are March Acceleration items available in the item store for diamonds. you can also just buy them as you need to use them, while the APC is marching.

You need to have researched the NOS item in each APCs research tree in order to speed up that APC's march.

You cannot unfortunately use the sped up on all march types. When your AP is marching tap on it or on the march indicator that appears in the top left to see if you get a 'Speed Up' option to let you use the march acceleration on it.