Since you've just given the model of a tablet and not actually said what version you're compiling it for, I'll assume that you mean Diablo, which is the latest OS2008 available for the N800.

For Diablo, at least, libsdlttf can be found here: http://repository.maemo.org/extras-d.../s/sdl-ttf2.0/

So:

Adding deb http://repository.maemo.org/extras-devel/ diablo free non-free to /etc/apt/sources.list and then running fakeroot apt-get update && fakeroot apt-get install libsdl-ttf2.0-dev should work.