I thought I had a second key, apparently I was wrong. Can I get a blank key online and take it to a locksmith to have a duplicate made? It looks like it has a chip comparing its appearance to those that do so does it need programming also? Can the right locksmith do that too?

The new key will need to be programmed to the van, but for an '04 most mobile locksmiths will have the stuff to do it in your driveway. You can use a non-Nissan blank and have the smith trace it, but be careful about crappy blanks. Some of the junk on the internet is really crappy, and most older Nissan ECM's will commit hara kiri if they get too many bad codes from a weak key. It's not a pricey item, so get it from a reputable source if not from the dealer.

What is the difference between a blank master and a blank sub? Looking on Nissanpartsdeal.com.

For an '04 there's probably a "valet" key available, as well as a "dumb steel" version with no chip that will only work the doors.

An especially well equipped locksmith will use one of his own blanks and program the chip on your vehicle.
All you need to bring is one working key.
I had this done twice:
https://www.thenissanpath.com/topic20956.html