We need the substance which on heating gives extra very pure nitrogen gas, $N_2$.

Check each option:

A. $NH_3$ with $CuO$

The reaction is: $2NH_3 + 3CuO \rightarrow N_2 + 3Cu + 3H_2O$ Although $N_2$ is formed, the gas mixture contains water vapour and may also have impurities. So this is not the best source of extra very pure $N_2$.

B. $NH_4NO_3$

On heating: $NH_4NO_3 \rightarrow N_2O + 2H_2O$ This gives nitrous oxide $(N_2O)$, not nitrogen gas. So this is incorrect.

C. $(NH_4)_2CrO_7$

On heating: $(NH_4)_2CrO_7 \rightarrow N_2 + Cr_2O_3 + 4H_2O$ Nitrogen is formed, but water vapour is also produced. Hence the nitrogen is not obtained directly in extra very pure form.

D. $Ba(N_3)_2$

Metal azides decompose on heating to give pure nitrogen: $Ba(N_3)_2 \rightarrow Ba + 3N_2$ Since the only gaseous product is $N_2$, it is obtained in extra very pure form.

Therefore, the correct option is: $\boxed{D}$