Is it rude to ask a professor you do not know and in a different department to borrow equipment? I am a grad student in new lab and my PI like to pinch pennies a bit. I want to purchase an accessory for use with an already purchased piece of equipment. I have emailed a few vendors about getting free samples,but I do not think I will be successful. I was thinking about emailing a few professors based on their area of research and the high likelihood that they may have a similar product that will allow me to perform a quick trial and build a case for my PI. 

I would need to borrow it for like 2 weeks and I can autoclave it to ensure it is well cleaned prior to returning.  

Is this considered rude?
I don't think it is rude, exactly, but I also don't expect that many would be willing to accommodate you unless you can first establish some relationship with them that is mutually beneficial - exchange of ideas, for example. 

But one researcher expressing a need to another isn't really rude, though perhaps a bit unusual and unexpected as you describe it. 

But since you suggest you are in the same institution, it has a higher likelihood of success I would guess.