Nitro is wildly infeasible to inject (and stay) into water solution at home... you can scroll down a few pages for a thread on this where I also was looking for this. Besides all the negative feedback there, I finally posted some web sites that talked me out of it. They explain how it is done commercially with either super expensive equipment or with tricks that make a small amount of n2 seem greater.
What you may think it does, it doesn't really do. There are some tricks that happen only if there is also co2 in solution as well as the tiny bits of n2 that is possible to dissolve. You can't really significantly put n2 in water unless you use great cold and pressure. n2 can't easily liquify into a pressure bottle like co2 so is impractical to store large amounts at high pressure. You don't at all need it for chocolate foam, I have recently posted recipe for mocha soda that has a super thick foam thru the use of a bit of corn syrup.