Does anyone know of any simmers who include PSDs with their creations?

Not here are MTS. The package format does not support PSD format. You'd have to ask the individual creator if they have PSD's. Not everyone uses Photoshop. I do have Photoshop but I would not have the psd format because I work with the DDS, PNG's or BMPs directly.

A search of the Downloads section comes up with one creation that has a PSD (or, rather, an off-site link to a PSD): Panties!

I know I've come across a few like this, but not from any one creator.