the harbor freight 1x30 belt sander is small enough for apartment use, i would think. as long as you're not making a mess with the thing. the price tag is only $40.

after i hit the blade with that, i'll move on to ceramic rod or leather strop if i need extra precision. working freehand just takes time and practice. also, a homemade sharpening wedge helps, it sets the angle for you, you just hold the knife flat.