Hi @trevor ,
Adding realistic 3D models on your scene might slow down the performance and adds up to render time quite a lot. I personally think it's just not efficient for landscape/architectural visualisations (that i assume you're working on?).
I only recommend using them if people will have a direct interaction with your work/product at close range (using your Chair, holding your Mug etc.)
Here are some free models that you can get:
3D Warehouse - HQ Man - Although it's rare, you can find some decent ones in 3D Warehouse.
3D Warehouse - AXYZ - 3 model provided by AXYZ.
AXYZ - 2 of them are ready to import .obj to Sketchup. Others, you might need to open in another application and export. Also you'll notice some models that have weird pose (arms wide open), those are 'rigged' and can be animated. This is useful for using same model with different poses and obviously to animate. You can either do it in their software called 'Anima' or other applications (Not sure if you can do it in Sketchup). I recommend watching their demonstration videos.
Turbosquid - Lots of models here. You can search for 'Futurescan' models.
Render People - 1 business man&woman
Human Alloy - a female.
I used QuadFace Tools to import .obj to Sketchup. It may not work for some models, i'm not sure why.