Where in the Bible do we find Sunday worship to be supported?
I believe you put the question in a wise way. You ask, not “where in the Bible can we prove Sunday worship,” but rather where can we go to support the practice. The strongest support for Sunday worship comes from outside the Bible. We can conclude from the writings of the early Church Fathers beyond reasonable doubt that the apostles universally practiced worship on the first day of the week (also known as the eighth day by some) in the churches, although meetings on Saturday as well were not unknown.
Biblically, the evidence for Sunday worship is a bit limited. We have evidence in Acts 20:7 from the fact that the church gathered at Troas “on the first day of the week to break bread.” It is most likely that this implies a meeting of the church which included the Lord’s Supper and, obviously from the context, some preaching. Does this prove that Sunday was always the unique prescribed day for worship and sharing the Lord’s Supper? Probably that would be an overstatement, but it is an example to which there is no counter-example in the scripture (in other words we have no direct evidence in the Bible of the Lord’s Supper being shared on another day).
Another piece of evidence for Sunday-worship comes from Revelation 1:10 in which John tells us that he received his vision on “the Lord’s Day.” We know from external sources that this certainly was on Sunday, as this is how the term “the Lord’s Day” was always used. Of course, this is not a record of a worship service on that day, but it shows that an apostle already was using the common term used later to describe Sunday as a day uniquely and specially dedicated to worshiping Jesus, remembering his resurrection and taking the Lord’s Supper. This does not prove that the church in Ephesus met on Sunday to take the Lord’s Supper, but it quite strongly implies it.
As for Paul, there is evidence that at least in certain cities he met with the Jews in Synagogue on some Saturdays (Acts 13:14,44 for example). Although he was a Christian and was in the habit of regular Sunday worship with the church, he still took part at least some of the time with Synagogue worshippers on the Sabbath, some of whom he was able to convert to followers of Jesus.
On balance, the evidence from the Bible that the special day of worship was on Sunday is quite strong but not absolutely overwhelming. By the way, “Sunday” at least for the Jewish Christians, probably included Saturday from sundown to Sunday sundown, so worship in the evening of our Saturday would probably have been considered part of the Lord’s Day, at least in the very early church.