I just discovered this site this week.
You can buy all sorts of K'nex parts and accessories separately: which is a godsend to us.
DS2 asked for more K'nex for his birthday, and I was shocked as we have mountains of the stuff! But after a little delving I discovered that what he really wanted was a few very specific parts; so I've been able to order exactly what he wanted.
Still managed to spend a fortune of course! And it hasn't arrived yet, so I can't vouch for their customer service.