I'm assuming you have these in the US? But in the UK, there are loads of charity shops where you can buy cheap books, clothes, DVDs etc. Anyway, if you look in the book section, you can always find really good books on the kind of stuff that make great content ideas for niches.
For example, there are always books on dieting, golf, fishing, making money and so on. The great thing is even though these books are often a few years old, most of the information in these types of books are evergreen. Also, as the text is most likely not online ( at least word for word), you could basically read a passage and re-write it in your own words, and there's your content, which was originally written and researched by an expert.
Anyway, hope that is useful. Failing that, just buy other random stuff for the hell of it, because it's cheap lol I just bought a set of 5 80s music CDs for 50p lol