Everything open box that I've bought came with all of the parts. In the case something doesn't, if the manufacture has good customer support they'll probably be able to help you out.
Fractal Designs replaced a broken fan controller for me with no hassle (they paid the shipping and didn't ask for me to return the old one.) I had a similar experience with Noctua where I lost the included low-noise adapters (they could have charged me and made me pay for shipping) but they sent me new ones free of charge. If it's from a decent manufacture they'll probably be able to help you out, at most you'll have to pay shipping. No decent company is going to charge you for a couple of screws.
Edited by Ecstacy - 9/13/13 at 1:25am