You can purchase EndNote which is very expensive and acts as a plug-in to Pages.
You can also use Pages' Footnotes/Endnotes:
Inspector > Document > Footnotes & Endnotes > Use Footnotes/Section Endnotes/Document Endnotes
When you want to add a reference, click in the text where you want it:
Menu > Insert > Footnote/Endnote
It will appear at the foot of the page, end of the section or end of the document, whichever you chose above.