Build a base of knowledge about your topic. Full-length books provide the most in-depth coverage and perspective over time, but an encyclopedia entry or book chapter will often provide sufficient background information.
He rang'd his ropes, and preach'd up patience; Back'd up his opinions with quotations. -Matthew Prior, "Paulo Purganti and his Wife"
Quotes are widely available on the web, but consider consulting a book (or an online book through the library) to dig up an obscure quote. The print books have the advantage of having very thorough indexes to help you get the perfect quote.
Speechwriters find that statistics are an essential part of winning any argument. However, 34% of statistics are made up on the spot (source? HA!) Follow the links below to get accurate numbers from authoritative sources.
If you want to relate the occasion that your speech is meant to commemorate to a current or past event, try searching news sources to get an overview of those events.