It's not simply a matter of commercial use that you need to check, but whether a font's license allows embedding or not. There are many fonts that allow commercial use, but do not allow embedding. In the case of those fonts, you either need to get a license type that allows embedding (which is hugely expensive by the way) or be able to rasterize the text or convert it to curves.
As I do freelance cover art, I buy a lot of fonts and the sites I would recommend for buying fonts licenses if you want to go that route are
Creative Fabrica,
Creative Market, and
Design Cuts. Creative Fabrica also has a very large free font section which is reputable. The fonts on these sites do allow commercial use, but do not allow embedding without a special license for it. However, they can still be used for the interior of a book if the text is rasterized or converted to curves.
A lot of your final font choice will be done during formatting. Some hire out for this, but it is also possible to do it yourself. I hired out the first time, but did it myself for this last book. If you want to do it yourself, I would recommend Affinity Publisher for paperback formatting (not recommended for ebook though as it can't export in epub format, for that Atticus is better suited and it is pre-programed with specially licensed fonts) if you want to avoid an Adobe subscription. One of the reasons I recommend it over just formatting in Word (which can be done) is that it allows you to convert a specialty font to curves (or rasterize it) which is something that Word cannot do (at least not last I knew anyway, it could only do embedding). Converting to curves does make the file larger, but generally speaking you wouldn't use a font that doesn't allow embedding as your main body font, but for smaller sections of text (chapter headers, etc.). I used a combination of Google Fonts and purchased fonts for my book so that I wouldn't have to convert everything.
If font licensing really worries you, I'd recommend sticking with Google Fonts for now. They are all free for any usage and allow embedding so are completely worry free. Be very, very careful with other 'free' font websites though. While some fonts on those sites are legitimate, many of these sites have a piracy problem. DAFont (or any of that guy's other font sites as he has multiple) is especially a website to absolutely avoid as many of them are stolen.
