@Atmos this post is a bit old ss below for this link on Reddit
The sensor os designed by Valve but manufactured by Triad Semi conductors.
https://triadsemi.com/steamvr-tracking/
There is a post on here somewhere before that had link to buying them iirc during the KS days.
The tracking sensor chips was TS4231. No longer showing directly on Triad page.
Original post on it. But Lightbox no longer shows info in the screemshot and the link doesn’t goto the direct product anymore.
@Chumet might know the cost though or your pretty good at finding BOM lists. 
I would suggest though comparing say a pimax BOM with those chips vs say a Valve/HTC product in case of difference in prices.