You can get either three ninjas in the Hokage Optional Box, where it gives out the frags for one of these hokages. You c*so get him in random events in hot topics in a form of frags. Lastly, you can get lucky by getting one of these ninjas if they show up in lucky ninja packs (Lucky Board), mission general boxes, or lucky wheel. However, I recommend you not waste you coupons trying to get these ninjas in the last point since its only based on luck and you are NOT GUARANTEED to get the ninja. Here are more easier ways to get the 1st hokage/2nd hokage:

1st Hokage: You can get him in Tendo treasure for every 400 pulls at a 50% chance since there is two super rares which is Pain Tendo or 1st Hokage

2nd Hokage: You can get him if you have level your battle armour to level 9 only in a battle armour rebate (which happens every 4 weeks).