Hi PeterM22,
I think one of the best way to find long tail keywords is from adwords by setting up a campaign using "head keywords" in a broad match. Run it in a week or 2 and you'll see a list of possible long tail keywords in "Show all Keywords" section.
Other than that you can try the the following options/tools to find more long tail terms;
- Semrush -
- Keyword Discovery
- Google - Search base keyword tool
- Google Trends
- Google Related Search
- Google AutoComplete
- Bing Related Search
- Bing Search Suggestion
- Ask Related Search
By the way - (some of the tools that I mentioned above are paid which I have no affiliation whatsoever)