Well my only two resources is Daz3d and Renderosity as they both have a level of expected quality as to fit to finish but there is of course what would be considered general use assets that can be made to fit the base G3 form.
For example I buy most (more or less) all useful animations from the Epic market place and re-target in MotionBuilder on to the G3 framework we are using in our project and in Unreal 4.
Don’t see why retargeting could not be done in UE4 and made available as a derivative. For example the Epic free animations a set/version could be made very easy for G3.
Still
It’s not all that hard to make something fit or conform to the framework As Daz Studio does have the tools to do the job. Might be why Iclone can import a G3 model. 