How do i get a Nintendo network id to use pokemon bank on my modded 3ds ?
Just make one. Use Nimbus to make sure your on Nintendo and not Pretendo when using Pokemon Bank. You can still make a new ID and it will work fine as long your region matches the 3ds.
IIRC Pokemon Bank cannot be obtained now, you had to have purchased it (for when it was a paid service) or obtained it when they made it free on the eShop, downloading it now is impossible through the eShop legally