Some time ago I found this site: d-maps.com (http://d-maps.com).
There are many maps what can be used as base for preparing maps for Simutrans - but I found that some rivers are depicted wrong and some important rivers are missing on maps of Europe.
1. wrong depicted river - on this map (http://d-maps.com/carte.php?lib=europe_map&num_car=2229&lang=en) Labe (Elbe) is shortened - and only to its confluence with Vltava (Moldau) - what is depcited right.
2. missing rivers - on this map (http://d-maps.com/carte.php?lib=czech_republic_map&num_car=2510&lang=en) there are missing rivers - Sázava and Ohře - both rivers are well depicted on this Simutrans map of Czech republic (http://maps.simutrans.com/maps/cz.jpg)
... and may be that you will find some more bugs there - but for most maps it would be good.
Well, unless I'm very mistaken I would not call them heightmaps, as they don't show any heights. They are nice topographic maps though, but not really fit for Simutrans. Unless you are living in the Netherlands, where almost everything is flat...
I know that those maps are not perfect - because they are not heightmaps. But better something than nothing?