The license you apply only covers the content you created. If you incorporate someone else’s content, you should make clear that it’s not your content. Without making it clear, your readers might wrongly assume that the permissions you give for your content also apply to the third-party content.
Relevant Creative Commons’ FAQ:
May I apply a CC license to my work if it incorporates material used under fair use or another exception or limitation to copyright?
Be specific in your license notice
A website consists of many different parts that could be licensed, so you should ideally be specific for which part the license is intended.
And you should note that not (necessarily) all content on the page falls under this license.
Your license notice could start with something like
Except where otherwise noted, […]
and include which parts are licensed, e.g.
content on this site is licensed under […]
this blog post is licensed under […]
images on this site are licensed under […]
In addition, you could also mark up in a machine-readable way (e.g., with RDFa) which content falls under CC BY-SA.
Mark third-party content
Ideally, your readers can identify the content that isn’t subject to the CC BY-SA license, and possibly also learn what they are(n’t) allowed to do with it.
To identify the third-party content, you could, for example, provide notices next to that content, or you could reference that content in the footer (e.g., next to your license notice).
Depending on the nature of the third-party content, you might of course have to meet some requirements:
If the third-party content is licensed under a compatible license, you’ll typically have to provide an attribution.
If the third-party content isn’t licensed under a compatible license, but its use is allowed by law (fair use, public domain, quoting etc.), follow that law’s requirements (if any).
If the third-party content isn’t licensed under a compatible license, and its use isn’t allowed by law, but you have got that party’s permission, follow their rules (if any).
For some best practices, see Marking third party content in Creative Commons' wiki.
Embedding third-party content
Depending on the parts your license notice refers to, you might not need to mark items as third-party content at all.
Note that the existence of a free/libre alternative is irrelevant for deciding if fair use applies.