The reason I asked the question was because autohausaz.com shows the '91 318is as an E36, and so while going to that website and picking 318is yielded M42 parts, it also yielded E36 specific parts. I too thought that it was only E30's in '91, so I emailed the parts department to see if it was an error.
This is the reply I got.
In 1991 BMW produced two 318is models, one on the E30 chassis and the other on the E36 chassis. While both were available, most 318is vehicle sold in the US were E36 chassis. This is while our catalog displays the 1991 318is as an E36 chassis vehicle.
I was hoping I could get some data to show him that it was E30's in '91, or, if E36's were in fact more common, just to know.
I've been going under the E30 318i (not is) to search for parts now, unless I know it's a 2 door specific item. Of course, you can always just search for part number and get exactly what you want just as well.
*Edit: Spelling mistake*