Funding sources depend dramatically by the historian and their "day job." For example, I work in a university, so I have various places I can apply for funding, including but not limited to: government sources (the National Endowment for the Humanities, or even the National Science Foundation since they fund work in the history of science and technology), NGOs (usually requires a policy component in my experience, but there are a lot of them), libraries/archives (many archives and libraries have "fellowships" that help scholars visit their collections), or foundations (the Sloan Foundation, for example, has supported historical scholarship in the past).

If you are a successful popular historian, you might get an advance from a publisher or publication for that kind of work, but I don't think that's the standard model anymore (if it ever was). To put it another way, you probably need a good agent to pull this off, and a pre-existing track record.

Separately, I am fortunate enough to have a regular salary (because I'm a professor), and that includes some research funding. So I have a certain amount of money I can use for whatever purpose I see fit, within certain categories recognized by the university (research travel, conference travel, conference fees, etc., all apply; "vacation" and 'a new car" would not). Like most institutions, my university also has "internal" funds that can be applied for, if a project is small enough that it could afford it easily. Like most institutions, they prefer that we get (and encourage us in various ways to get) outside ("external") funding.

Is it an amazing way to live? I mean, I like the job. But most of it is not spent gallivanting around doing research; that tends to be a small part of the overall job. Almost all of my travel is not research related, but related to giving invited talks, and in those cases the host institution pays for the travel. Occasionally I can make such travel dovetail with a research trip, but that has to be a very generous alignment of the stars. Most of my actual time is spent teaching, writing up research, and doing the very boring work that is required to make these externally-funded projects successful.

Separate from travel, there are a variety of ways to "get paid" in this line of work. Mostly it is not about the sales of a product, though, but some variety of the above types of funding sources. There are also, occasionally, contracting opportunities (my two most lucrative have been as an advisor to a television show, and being a guest curator at a museum).