The easiest way is to edit the profile, and then add your source after the line that says <references/>. You would start each source with an asterisk (*), which will look like a bullet, then type the source. You can also embed them as references, or footnotes. That is explained in the help:
https://www.wikitree.com/wiki/Help:Sources
Once you add at least one source (more is better!), then you can remove the {{Unsourced}} from the top of the profile, which will get rid of the orange banner saying that the profile lacks source information.
If you need to find a source, I recommend trying RootsSearch. If you look at the right side of the profile when you aren't editing it, you'll see the link under the "Research" heading.