For what it's worth, there are isometric tilesets available for Roll20. A search for "isometric" on the Roll20 Marketplace turns up several results, including Isometric 3D Desert Tomb, Isometric Sewer, and Isometric Dungeon (by Plexsoup; I am not affiliated).
The description pages for Plexsoup's isometric sets also includes instructions for creating a fairly seamless isometric experience within Roll20:
Tips for optimal experience:
- Turn off the Roll20 grid
- Make a new page
- Set the Page Size to 96x96
- Set the Scale to “"1 unit = 7 feet”"
- Set the background to a nice, dark color
- Go to the Map layer
- Start at the top left corner and lay tiles from back to front
- Right click on tiles and set “"To Front”“ or ”“To Back”“ as required
Notes about isometric or side-view play:
- The ruler will only show proper distances along cardinal direction. ie: diagonals will show the wrong distance. Counting squares still works fine.
- Dynamic lighting requires special care around doors, because you want players to see the door.
- Character Facing is less obvious than in top-down play. Rotating side-view character tokens will make them “"lie down”“. Great for showing casualties! To change facing, right click a character and ”“flip horizontal”"
These instructions can assist you in creating your own isometric maps, potentially even for free if you can find appropriate art online or you have the necessary art skills yourself.
For a different "3D" experience in Roll20, the marketplace also has Dungeon Depth Tiles and Dungeon Depth Tiles Supplement I (by Stephen Shomo; I am not affiliated). Rather than being an isometric view as Plexsoup has achieved, Stephen has created a top-down view of a 3D environment, so that nothing needs to change for the behavior of your tokens, dynamic lighting, measurements, etc.
If you have access to a Mentor subscription, Stephen has also been working on a dungeon generation script using his Dungeon Depth Tiles set. The way his tiles are designed also makes dynamic lightning simpler: