I don't think "Two Brothers" makes the single side, low mount full system (header and can) anymore. Their website only shows the slip-ons for the standard 919 header system, or the "stash" which turns one of your mufflers into a storage canister.
It may be hard to find a used 900RR header, but not impossible. They still show up on ebay from time to time. Some motorcycle graveyards may have them. I found one on ebay a few years ago and mated it with a little known carbon fiber can (Wileyco) and for less than $200. I have a system that includes the exhaust setup, a PC3USB with a good map, and a 17T counter sprocket, all working well, so I've never looked back.
I took the used header to a shop that sandblasted it so I could finish it with a rattle can of high temp flat black automotive paint. It looks good to me on my silver 2004 919. Good luck.
I won't comment on the quality of Two Brothers but I'm sure others will be along shortly.