Dyson has over a thousand maps in a black and white line art style. Sly Flourish has a nice article on dungeon maps that has links to many map creators.

Generally I just Google the theme I want with the word “battlemap” and tell Google to only give me high resolution results.

Two Minute Tabletop has some great stuff

Czepeku has some really beautiful maps. Here is the link to their free stuff, and the lowest patreon is only $1 if you are so inclined

https://www.patreon.com/collection/43813?view=expanded

Maybe not directly related, but you can use Tiled to create your own maps. It doesn’t export the finished map as an image, but you can probably use the PrintScreen key to work around that