As a font designer, Monotype is the biggest and best. They have all the top sites: fonts.com, myfonts.com, itc.com, and several more. They are the sites used by the pros, primarily. At the least, the pros sell their fonts there also. These places are where my fonts are sold, in most cases.
Ebook licensing is very new
I have the entire Adobe library and several others, maybe 4000 fonts total. None of them are licensed for anything other than print.
In fact, until 3 or 4 years ago there were basically no fonts with an ebook license. The problem is that ebooks are just small zipped websites with the fonts sitting openly in a folder when the ebook is unzipped and you look at the pieces.
So, the ebook license assumes that you are giving each reader a copy of your fonts. The pricing is the same as OEM pricing, where a font is included free with purchased software. That is, in fact, what happens when you buy an ePUB.
Adobe encrypts the fonts in ebooks exported from InDesign, so ebook licenses have gotten a lot cheaper. But they are not encrypted by Word or any of the other word processors, as far as I know.
Fonts are used in outline form: OpenType or TrueType these days. Once you use them in a Photoshop file, you convert them into pixels and the outline data is gone. They are no longer fonts at that point, so you can use anything you want.
I’ve never seen a free font with an ebook license. So, read your license carefully.