You may want to look into the viscosity of the oil used to suspend the N in the vial. If the gague is too high and it's a thicker solution it might be a struggle to get it to pass through the needle and inject easily. The oil type should be indicated on the label of the vial if it's from a legit compounding company.
For my weekly medication injections I use a thicker 18G needle to draw up the medication into the syringe and a smaller 23G needle to inject the solution, so it doesn't get contaminated or dull the tip from the rubber membrane. This also helps the process go smoothly with minimal resistance. In my case my medication uses castor oil so I can use as narrow as a 25 gague needle.
I'm from the states so you can just order them from amazon as others mentioned here. Not sure how that would work over there.