I can't find stock anywhere. Are there any other solutions or have I messed up?
According to your case's site, it has a button on the front that controls the built-in RGB strip. Also, that strip is expandable, so you can add other 4-pin strips or fans in the chain and they will all be controlled by that button.
If you want software control of your RGB, you will need an RGB controller (e.g. NZXT Hue, or that Deepcool thing
@ScuBaLlama linked) or a new mobo with an RGB header (in this case, a 4-pin header).
Some motherboards have 3-pin headers, and some RGB controllers work with 3-pin connectors, but then you won't be able to sync the RGB on your 3-pin devices with the RGB on your 4-pin devices. This seems to be a common issue people have, and that Deepcool adapter seems to be the only straightforward solution.
All in all, right now, if you really want an RGB fan, your easiest option is probably to get a new fan that has a 4-pin RGB connector and connect that to your case's RGB strip. Keep the 3-pin one and stick it somewhere less visible. Then you can make a more coherent plan for future purchases (e.g. getting a motherboard with a 4-pin header, or an NZXT Hue and all NZXT fans/strips).