Although there were Unitarians that predate Russell, the origins of the non-Trinitarianism of Adventism came directly from the Christian Connection ( which later merged with the United Church of Christ). Many from Christian Connection became Millerites and brought their Arianism with them. Russell likely got his Arian views from George Stetson, who wrote a series of articles about the Ransom around the year 1872, in which he wrote that Jesus was a perfect man, a corresponding price to Adam, and had three different natures, pre-human, human, and post-human. Russell's view of the Ransom conflicted with the Trinity. George Storrs was ambigious on the Trinity, preferring to stick to Biblical language, but held similar ransom beliefs, but not as well defined as Stetson.
There were similar ransom views among the Adventists, and Stetson was likely not the only one to attempt to explain the theory of salvation in non-trinitarian, Arian terms. One is J. M. Stephenson (search online). Arianism was prevalent among the Adventists at that time, and openly espoused. However, SDA officially adopted the Trinity in the 20th century, as did the Advent Christian Church, and Herbert Armstrong's group Church of God. The Church of God 7th, an Adventist group, recently adopted a binitarian view. The Church of God of the Abrahamic Faith are non-Trinitarian, but are not Arian since they deny the pre-existence of Christ, influenced by the Christadelphians.
Russell's views are similar to unorthodox Christians in England around the time of Isaac Newton, John Milton, Biddle, William Whiston, with their date-setting, apocalypticism, Arianism, conditional immortality, annihilationism.