How can I easily find internal link opportunities?

I'm writing content for a client with loads of existing blogs and landing pages targeted towards specific content.

I'm currently using "site:clientssite.com "keyword" search modifiers to find link opportunities, but it's really taking time.

Is there some form of plug in or add on for Google Sheets that can easily crawl a site, crawl my copy and point out internal link opportunities?

Link Whisper pro.
It does the internal linking automatically and the relevancy is more or less passable.

The free version shows link building opportunities in each article but you have to insert them manually.

Link whisper is the way to go. I won't suggest auto linking, you will end up with lots of unnecessary and low quality , irrelevant and same anchor text links.

I would recommend Link Whisper. You can quickly add internal links to multiple pages or posts simultaneously. And it offers anchor text suggestions while creating or editing links.