Is there a way to find a used sub that will connect wirelessly to my vizio sb3621n e-8 soundbar? Or anyway to wire one directly to the soundbar. I understand it's wireless. I bought a used one and it works great just needing sub
Unless you have some electronic skills you will need the subwoofer that came with the soundbar.
If you can locate the wireless transmitter for the sub inside the soundbar you can run a cable to its' input. Might be a module but more likely to be a chip.