SH3D is great at easily modelling and visualizing buildings and their interior. It is not trying to be a generic modeller. This limitation is really its advantage, as the tool can be kept simple this way.

That said, I figured 3 ways to obtain models for interior objects, in SH3D generally called furniture:

1. 1. do "real" solid modelling in a CAD like software like FreeCAD
   2. make meshes (no solids and not parametric) with textures in a VR modeller like blender
   3. find suitable models on respective web sites. I guess searching this forum or googling for 3d furniture models gets you there.

I’m also looking for a cheap (free) 3d modelling program more in the sold modelling

#1 takes the highest efforts but may pay off for you. If you don't want to spend a fortune, the next powerful CAD system is FreeCAD (FC). Runs on anything that has mouse and keyboard, free as in free beer and as in freedom, constantly improving.

Cost:

1. - steep learning curve though milder than blender's,
   - usability not quite state of art of commercial systems
   - models have close to no visual properties, at most color and alpha per face.

Reward:

1. - models suitable for prototyping, e.g. NC machines or 3D print, for designing own fittings or accessories based on the existing; accurate to the nm (10⁻⁹ m) if you wish
   - solid models, i.e. no worry about face orientation, tesselation after modelling, i.e. not part of the model
   - parametric design, i.e. every modelling step can be parametrized and changed any time back in history.

If you feel like going that way, I am ready to support you, as are many more people. Here with the basics or with model interchange among FC and SH3D, more in depth in the FC forum .