Hi I'm looking for a height map of an real life terrain, to create a terrain out of it.

Loops

Hi, I'm looking for a height map of an island in Greece, to create a terrain out of it. The thing is I have tried a lot of different websites but can't get a good resolution. Is there a way I can create my own heightmap with a drone shot or something like that?

A_B

I seem to recall a way to export .obj from google earth. If the island you're talking about has a decent 3D representation on google earth (much of the world does) then you may be able to export from there and create your height map off their geometry. You can google as well as I can!

Other than that I know nasa has a massive geospatial data store of pretty much all the elevations on the planet which I'm guessing would be accessed through here: https://earthdata.nasa.gov/

Alex_J

None of the height map data you get online will have very fine resolution, like 1m per pixel. I don't think it exist. 

What you can do is start from the internet height map, put it into an app like world creator or similar and then run erosion filters over that to get more fine detail. Can think of the original height data as the primary and secondary forms of a sculpt, and the filters you add is just for pore level details.

poopipe

Photogrammetry with drones works quite well and it's commonly used for medium to low detail capture (up to a couple of metres per pixel).  I would imagine there'd be a fair bit of trial and error involved and it could get quite expensive - not to mention time consuming - but it's a valid path to follow if you have the resources. 

most Satellite height data is actually built from photos - there are lidar carrying satellites out there but the bulk is derived from stereo/tri-stereo photography.
This is basically just photogrammetry but the datasets are fucking massive and you need serious hardware to even contemplate generating 3d data from them, let alone removing trees etc to get the terrain model. 

if you want consistent high-res stuff of specific locations (better than 30m per pixel) you'll have to pay and you're looking at minimum orders in the thousands unless you can track down an agent/store that will do small bundles (worldDem data at 12px/m is very cheap at a few dollars per square km)

Alex_J's suggestion is almost certainly the best option for anyone without a corporate bank account to draw from

I'd recommend you download QGis so you can stitch and process what data you do find easily. 
one thing worth mentioning is that the data you get is very unlikely to give you accurate distances from pixel to pixel without being projected into the correct reference space  - if you're concerned about accurately recreating a real world space this is a subject you need to read up on. QGis has the tools you need to sort this out but the exact things you  need to do are dependent on location.