I just got a 1972 Datsun 531 1600 truck and I need to get a new key set made. Are local nissan dealerships able to cut a set for 72? or do I need to go to ebay and buy a keyblank and have a lock smith cut it?

Just go to the key shop with both of the lock cylinders removed and ask if they can help you, if they can't, ask what key number/code is needed for that lock cylinder and then call around to other key shops to see if they have the key needed, if they don't then start searching ebay.
Unfortunately you have to remove the cylinder from the door, there really is no other way.
What I did was collect sets of door lock cylinders from trucks in wrecking yards over the years and put them in baggies for future use, so they were already out of the door when I needed them.
I had one lock that the keys cost me over $7.00 each plus the cost of cutting them.

I have a key set already so all I need is a duplicate set made. I found some key blanks on ebay but they are coming from thailand.. other local sellers only are selling the door key blank or an ignition key that is for newer datsuns.

Uh..Hi guys! All you need is a screwdriver, or a nail file, or most any key. On my 71 and 72 521's !! Have been meaning to ask if the locks can be replaced with newer ones that are not so easily opened. I keep nothing in the cab, and use the old bar with a lock, on the steering wheel. Have read that you can remove the lock out of that thing by spraying some stuff that will make the lock smaller, and you can even remove that cylinder from the bar. You make it so cold, it shrinks, and can be removed if you act fast enough....supposedly....I can take most any old Chevy key, and unlock the 521's. Years ago, I did replace some of the locks, but never realized they could be opened so easily. I can also use a pocket knife. If someone has found some cylinders that are not so easily opened, I would like to get some too. One lock, I have to play around with for awhile, but it will open too. Without a key. These things are so old, I doubt people now a days even consider the possibility of opening a door with a finger nail file or a pocket knife! Or a screw driver. Even the tow truck drivers and cops use those bars to slide down the windows. When all they need is a finger nail file! The old 'Ford coil' trick will keep people out!