It’s pretty easy to draw real, proper, to-scale maps that are good-enough-for-behind-the-screen. You don’t really need any other source. Sometimes when I’ve been wanting ideas, I’ve tossed out items randomly (keys, blocks, dice, bottle caps), either physically or digitally (with the “pull shapes” mode of the old “Alchemy” java drawing app), and based the layout on that.
I’m not great at it (I did the “Duchess/Lichess” one page dungeon) but good enough for using at the table? Sure. And, your own stuff is always gonna be easier to use since you can understand your own scribbles.
Using a dungeon map from a module that the party has already visited, or is never gonna visit, also works. You can flip left with right or up with down, you can remove and add doors, and you’re changing the monsters, traps, treasure and scenery. That old “Hero Quest” map stayed fresh for dozens of scenarios just by having movable doors and furniture. Good layouts never die.
There are cartographers that make maps that are empty and ready for you to fill up with what you want. Dyson Logos comes to mind.
Remember to save it or print it. I’ve messed up by losing random app-generated maps after they’ve become canon. They are irretrievable.
These are small pieces of maps made to fit together in many ways. While I mostly use modules or mini-dungeons (like One Page Dungeons, Mini Dungeons, or Trilemma Adventures), when I do need to throw together something custom or flesh out an unfinished seed from a module or adventure collection or generator, I most often use geomorphs. I make sure to record the generated map somehow.
These were also wonderful when I was DMing out of a box of A7-sized index cards. I could jam the geomorphs in there and I could record combinations of index cards on the index cards.
I grew up on these, coming from the interactive fiction side of things. These are maps completely removed from all spatial or scale realism. For example, you know that the hallway connects to upstairs and to the kitchen and the washroom, and that the kitchen connects to the dining room and that the dining room contains to the back patio.
But instead of drawing such a house, you draw a box and label it “hallway” and draw lines to boxes labeled “upstairs” and “kitchen” and “washroom”.
You can use just dots for the rooms, or you can use boxes big enough to list monsters, stats, treasures, and traps right there on the page. You can also scribble next to the lines themselves if the connection themselves are significant (a locked door, a hallway with graffiti, a puzzle).
While these are great in the programming, linguistics, IF world, I’ve fallen out of love with these in favor of the other types—the more visually accurate maps, especially those that are so detailed that they show mats, desks grates, chairs, traps, your Schley maps, your Prescotts, they give a DM so much to work with without having to slog through text—but flowchart maps have two huge advantages which means there are situations where they are the unbeatable best.
Both advantages stem from the fact that flowcharts don’t have to differentiate between up, down, south, west, in, out, small, big, dream, awake, a flying carpet ride away, etc. Things can be wildly connected. They’ve also been used to map relationships between people or entities for that reason.
This makes them great for three dimensional structures, like the underdark, which is why Veins of the Earth uses them.
This also makes them amazing for mapping. They are great—still the best—for when playing a video game, especially a text adventure. When you’re not the DM, when you’re just post-hoc trying to make sense of information you learn piecemeal, these are the unbeatable GOAT.
But when you are the DM, and you’re working from a position of having all the info, there’s something that has these flowchart maps beat:
These aren’t as well known, so I have a separate post explaining how they work.
These have the potential to have all the abstractedness of flowchart maps, or all the spatial logic of carefully drawn maps, or anywhere in between, and the rooms come pre-numbered with the handy-dandy grid coords.
They can’t work when you’re an explorer mapping completely post-hoc (“whoops, I should have made room for an entire second floor here” or “wait, what, there’s an extra room between the armory and the quarters?”), and once you’re polishing your dungeon up for publication it might be more appealing to draw it up in detail, but for your own DMing, they’re awesome.
I actually rarely use them (I mix and match all the methods on this page; three seconds of dividing a sheet of paper into “rooms” and adding doors and stairs have made a fine “ruined castle” at times) but whenever I remember that diagram dungeons exist, I’m like “oh, yeah! they’re the best!”