Most of the big companies such as Herman Miller, Haworth, Steelcase, Knoll all have online stores where consumers can buy individual items. Those brands also own a bunch of other brands such as Muuto, Hay, Naughtone, turnstone, Coalesse, BuzziSpace, and more which could have some crossover into the home office setting (and can also be shooped for online).
I imagine as the consumer market for home office heats up other brands will figure out how to manufacturer limited lines with limited options for finishes and fabrics so they can be made and shipped more rapidly.
For C9 though, you actually don't need the rail (we have 3 single desks in our office without a rail). While these are more expensive than something you could get elsewhere, they are built like a tank and manufactured to order to the specs you need at a facility in Washington State.