The first entry of the Ultra Series, Ultra Q, featured mankind encountering the giant monsters known as Kaiju. The series was released in 1966 by the special effects studio Tsuburaya Productions, which was founded in 1963 by special effects wizard Eiji Tsuburaya, the man who brought such monsters to life as the King of the Monsters himself, Godzilla.
Alternatively, you can also find some of their releases on their streaming service movieSPREE if you are in the United States or Canada.
We also recommend the YouTube channel Ultraman Explained, it is a small channel, but it is growing and it is one of the most resourceful places to find Ultra Series informative videos. In fact they use this Wiki as one of their sources!
Please check out their videos, and if like their work maybe you could subscribe to their channel and help the Ultra Series community grow.