I bought mine through awesome gti, vagbremtechnic are 1 of their side projects they have going on there.
You can go down the route of rs4/Macan/q5/rs6 etc callipers and discs and save some money. Although your stepping up the number of pistons and possibly getting a slightly larger disc they are not going to be much of an upgrade over standard brakes for repeated stops from speed. With Oe brakes from another model your never going to get fade resistance like you will with a proper track setup from the likes of stoptech/ap racing etc and if you push them hard enough they will fail you!
My discs are 390mm and (I think)36mm thick with separate bells, they have way more veins in them to aid cooling, I’m running ds2500 pads, braided lines and uprated fluid as well.
Other than awesome gti there’s a spot I’ve looked at called Reyland motorsport that do some nice looking kits. I think they use their own make of disc rather than ap racing unless you specify you want ap. Another place I know of that looks really good is Tom wrigley performance.
where you are in the country will probably be the biggest factor on where to go?