The steps that you should take generally is:
- Get elevation data - (they are rasters available in different formats and resolutions)
For a finer and more accurate map, you'll need more resolution or in other words smaller cell (pixel) size! Digital surface models(DSM) are generally more preferable for your case than digital terrain models(DTM) or digital elevation models (DEM). So look for free DSM data with a resolution of 1m which means in every sq meters you have an elevation point. Typically there is no free data for DEMs or similar with resolution (cell size) 30 meters (1 arcsec) or less. Search and you will find.
- Install QGIS and load the raster
It's a free and open source GIS.
- With the Contour tool in the toolbox select your input layer(the same raster)
- Fill "additional creation parameters" with "-3d" to force it 3 dimensional.
- Right click on the layer, Save as, and save it as DXF in a directory and you are ready to go.
Another thing worth mentioning is to keep an eye on the projection system. Most of the digital models are in WGS 84 so if you are working in a metric unit instead of angular, convert your projection at step 5.