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Are there couplers out there? Maybe you could pair two (or more?) six-footers to get the length you need. Not sure what the maximum feasible length for USB-C is, of course...
 
What is your projected use for a longer cable? There may be other solutions, depending on your needs...
What do you need to connect, (that can't be moved closer...) (?)
(Too-long USB cables can have timing issues, or voltage loss if you need it for power/charging.)
 
In Japan it's fairly easy to buy USB-C cables in 3 meter and 4 meter lengths. I haven't actually bought one, so I can't say whether or how well they work. My issue was distance from power outlets, which I solved by buying a 4 meter figure-8 power cord instead.
 
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