You have to look for the "3D printable" ones there - not many of them are!
There's another site (competitor to Shapeways) that starts with 'P' and ends with 'onoko' where you can purchase some as well.
...or you can hire somebody here in the forum if you still can't find what you need!Pssst! I wrote a book to help you quickly get your Sketchup models ready for printing. Click here to get it on Amazon
To get a good answer you'd have to expand on what you mean by commercial use .
One place that is new but growing steadily is 3D Printing Model. They deal specifically in 3D models for 3d printing. However, If you wanted to do something commercial with the models purchased from 3D printing Model you would need to make arrangements with the respective copyright owners.
Each site is different in the ways that you can use the models that you purchase. Turbosquid as an example doesn't let you contact the copyright owners very easily, so you basically have to abide by their EULA.