Just last week there was an event where you could obtain Edo Tobirama in Wheel of Fortune, though that event is highly RNG so you may not even get him after spending tons of coupons in the event (On the other hand you could get him really cheaply but odds are that isn't going to happen)

Unfortunately lately hes obtainable only in milestone rewards in events like great plates or as rng reward in events like lucky wheel. There wasn't any direct buy for his frags in like 2-3 months.

Theres no special way. You just need to wait for him to show up and then buy him. If you really want him start stacking resources already to get as many fragments as possible, since he appears quite rarely in good events.

He appeared in Konoha Tree event before.

18k+

Tree event isn't that bad when you are going to do all-in.

You can submit your query to them or let mods know???. They might put it through.

Hi! I will forward your feedback regarding Tobirama [Edo Tensei] for future events to the team. Thank you so much for letting us know =]

If you guys have any additional feedback regarding events, it would be very helpful if you can give us your feedback here.