Ok I have had to do a lot of looking because I wanted to have a beer gas set up, here are a few things I learned.
First of you are looking for "beer gas" or 70% Nitrogen 30% Carbon Dioxide Blend or 70% N 30% CO2 blend. You are not looking for Nitro, N2, or Nitrogen Dioxide.
The place I get my gas runs as an exchange not a fill up. I would recommend finding a service like this. I am able to exchange an empty CO2 tank for a full Beer Gas tank. Obviously I have to pay for the gas but they will take a tank for a tank. This is nice because if I ever want 2 CO2 tanks I can have that, or one Beer Gas one CO2 like I have now. This also makes it so I never have to worry about re certifying my tanks.
Beer Gas cost twice as much, CO2 refill is ~$10 Beer Gas is ~$22. You also "get" less volume, as CO2 will liquify at 700 Psi, I am not sure the PSI needed for N but it is very high. So CO2 will turn to liquid in a tank but the Beer Gas will not.
I have received tanks which are filled with Beer Gas in both CO2 and N tanks. CO2 tanks are only rated for I think 2000 psi or something close to that N tanks are rated much higher I think. Where I exchange tanks the Beer Gas tanks are always filled to 1500 PSI in Beer Gas.
The regulator I have can be used for CO2, N, or Air. You do need a regulator that can handle high pressure to dispense Beer Gas. N tanks also have a different threading, there is a converter for CO2 threads to N threads.
I guess if I was the OP I would find an exchange center so I would have the ability to use what I need.