I can't imagine there's much in the way of aftermarket parts, especially here in the US. We don't even get the full catalog of factory accessories let alone aftermarket parts like this to choose from. Most likely going to a dealer for an OEM part or importing a part if you want something aftermarket. I'd be surprised if there's aftermarket control arms anywhere, though, to be honest. A few years back it was sort of common for guys to replace the steel parts with beefy aluminum control arms on some cars. If I remember right, Subaru even offered them as a factory upgrade option for a while on the WRX when I bought mine. Not sure if anyone still does that or not. Probably have to order the part, but you should be able to get it in a couple days at most. They have to have some of these in the US for body shop repair work. My infotainment MCU module shipped from a warehouse in California and the dealer had it the next day. I bet they go through a lot more control arms than they do MCU modules.