Where should I start looking for sources on my topic?
In order to write your proposals, you're going to need sources to give yourself ideas and support your arguments. However, where you start looking for sources doesn't matter as much, as long as you know how to evaluate them. So, you might start with a basic Google search, or, for a little more context, you might try Wikipedia.
If you start with Wikipedia, make sure the text in the article is supported by a cited source. Also, make sure the source both exists (e.g. isn't just a dead link) and supports the claim made in the Wikipedia entry.
Okay, so I've found some webpages / articles / blogs, etc. Now what?
What you've likely found is what's called a "secondary source"1 on the topic you're exploring. That is, it's a source that discusses or summarizes for a broader audience the content found in one or more "primary sources".1 Primary sources, then, are those works which are authored by experts, are intended for a limited audience with similar expertise, and represent the highest authority on a particular subject. Don't be discouraged if you can't make sense of a primary source (e.g. a scholarly journal article or a patent for a device) on a given subject—this is what you would learn how to do in graduate school!
For the purposes of your proposal, your professor has determined that it's acceptable for you to rely on secondary sources only. However, not all secondary sources are created equal! What's important is to make sure you can follow the chain of authority from your chosen source to a primary source, whether that primary source is a peer-reviewed article, a scholarly book published by a university press, or even the quoted words of a noted authority on the subject. If you can't follow the chain of authority from claims made to expert sources of support, you probably should find another secondary source.
What if I see references to authoritative primary sources, but I'm still unsure about a secondary source?
Evaluating secondary sources can be tricky, and what can appear to be a legitimate discussion or summary of a primary source can turn out to be mere opinion, if not straight-up misinformation! How, then, can you tell whether you can rely on your source? A mix of critical thinking, skepticism, and vigilance will be your best bet, but there are also a couple of handy tests you can use to evaluate your sources.2
Ultimately, you will need to decide whether you think your source passes these tests and is therefore accurately representing primary source material. And, of course, whether or not you decide to include a given source in your proposal will come down to whether you think it will be convincing to your audience. If you cite a bad source, regardless of whether you were aware it was bad, you, yourself, run the risk of being labeled as an unreliable source!