First thing is to look at the instructions as to the requirements for wiring it. If it says use an appropriate plug, use an appropriate plug.

You install a box and use an appropriate fitting on the box for both the cord and the conduit.

If there is provision for the bonding wire, make sure you use the #8 solid wire to connect it to the rest of your equipotential system.

The instructions don't say anything about a plug. I just need a way to go from a cord to flexible conduit or at least to a junction box.

They make such things for junction boxes as I stated. You'll need a junction box because you're going to splice that cord to your other wires right?

You are likely going to have to go to an actual electrical supply house.

Big box stores rarely have anything they cannot sell a large number of.

Take the unit with you and ask them for a cable-box clamp connector rated for the type of 'cord' on the unit.

What about a fitting for rigid conduit that would allow me to pass a wire through it but be water tight at the same time? All of the supply houses near me are open while I'm at work so I need to find something on the internet.

"What about a fitting for rigid conduit that would allow me to pass a wire through it but be water tight at the same time?"

It is going to have to be made for the specific of cable.

While their is some room for the glands used to seal round cables, they actually have a minimum and maximum diameter they are rated to work with.

Keeping water out is not the only thing needed.
The cable must be clamped to prevent any force that might be exerted on the cable from getting to the internal connections.

If you use almost any cable clamp pointed downwards you will already have most of the rain problem eliminated.

Humidity is still going to get into the box, and it will condense.