Where can I find some sample logos for a clients module in a WordPress theme?
You mean logos to use without getting permission?
I don’t recommend to use any kind of commercial logo on your theme. Use the Envato ones (Creattica, FreelanceSwitch…) or create samples by your own.
Or contact any GR author who is making corporate identities, maybe he/she can share the previews for the cross link.
I actually just made a bunch of “Sample” logos for this exact purpose. It’s very sad when some authors have brands like Sony or Apple “endorsing” their pages, despite Envato policies.