I think I used #8 screws to attach a spacer like that onto the rear pods of my 2010 UC (both in the deep holes to mount the spacer to the pods and shorter #8s to attach the speaker to the spacer). I was able to find stainless ones at a hardware store in town.

I will say that once I had them mounted I didn't like how far they protruded into the passenger seat so I switched over to some 1/4" spacers and just used one set of screws to go through the speakers and spacers both.

 

I measured the length needed and went to my local hardware store. I found stainless steel self tapping screws which worked fine. Don't worry about posting too much people here are helpful and friendly for the most part.
Bill