I bought a pair of North Face hiking boots and I wear polypropylene or wool socks but my feet are just barely warm and it is only -5 celcius. I don't want to have an excuse when it hits -30C. I have read some people use some type of neoprene boot covers. What are these and where can I find them? I suppose another alternative is to shell out more cash for actual winter boots. What are the best bang for the buck? Someone here also suggested maybe using putting a plastic bag over the socks inside existing shoes to keep the wind out. What do you think?

Thanks in advance for the feedback.