The steps that you should take generally is:

 1. 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 another mean Less CellSize! DSMs are generally more preferable for your case than DTM or 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 are no free data for DEMs or etc with resolution(cell size) 30 meters (1 arcsec) or less. search and you will find.

 2. Install QGIS and load the raster

It's a free and open source GIS.

 3. With the Contour tool in the toolbox select your input layer(the same raster)
 4. Fill "additional creation parameters" with "-3d" to force it 3 dimensional.
 5. 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, keep an eye on the projection system. Most of the DEM, DSMs,... are in WGS 84 so if you are working in a metric unit instead of angular, convert youe projection at the stage no.5.

And by the way, welcome to the forum.