Because the egress windows from the basement will not be large enough for the fresh air requirement, I have to install a ventilation return duct from outside. How do I go about doing this? My local Lowe's only appeared to have exhausts, but no intakes. It also needs to be 6", so I can't modify a bathroom exhaust vent...

I don't know about houses but in factories and warehouses you need an MAU which stands for make up air unit. It's basically a fan motor from the outside but it has a heater for when the air is cold and some have AC units when the air is hot.

I don't need a unit with a fan, or a separate heater. This fresh air duct will combine with the return from inside the house and go into the heater from there. I just need to figure out what to put on the side of the house.