I'm looking for a text source of the Hay-Herbert Treaty of 1903 to cite in a paper, but all I can find is the wikipedia entry of it. Is there a more official source online where I can find the text of the treaty?
Do you have access to Hein? You can find it in Compilation of Treaties in Force from 1904. p. 386
If you have access to Proquest Congressional (formerly Lexis Congressional) search for S. Doc. 318 from the 58th Congress. It's also in Serial Set volume 4622. You can also find it at 32 Stat. 1961.
The entry at WikiSource is probably your best bet. I checked the history of the page and it seems like the text on the page was originally sourced from the website of the University of Montreal in 2005. That page has since been deleted, but it's still accessible through the Internet Archive.
The volume / collection "Foreign Relations of the United States" (abbreviated FRUS) undoubtedly contains what you're looking for. It goes back to about 1860.