ComiXology is a popular option that sometimes features 'Attack on Titan'. However, licensing agreements can affect its availability. Also, check if your subscription plan gives you access to it.
I'd suggest looking into the Crunchyroll Manga app. It sometimes features Attack on Titan. Also, keep an eye on official manga distribution apps like Shonen Jump, as they might have it. But remember, availability can vary based on licensing and your location.
The Attack on Titan manga is indeed done. It wrapped up, bringing an end to a popular and highly engaging series that captured the hearts of many readers.