This is a good starting point, IMO: <a href="https://marketplace.roll20.net/browse/set/430/dun" rel="nofollow">https://marketplace.roll20.net/browse/set/430/dun</a>... -- I like it because any tile can join with any other tile and there's a lot of GM-layer legends/graphics to help you remember where you have points of interest. Just searching for 'dungeon' will get you a lot of packs to look at, however.