That's a really sweet looking bike you got there... I'm going to agree with what the others have said about ThatsDax as I buy most my parts and all my engines there. SickBikeParts also has a lot of good stuff and they have the better throttle grip. Jake's has a lot of good stuff too but I really hadn't bought much from them, and I hadn't bought anything from pistonbikes for the same reason mentioned above.
For the disc brake, if your bike is already set up for disc brakes, you can get a hydraulic lever and caliper from here for $34 http://www.ebay.com/itm/111597851449...%3AMEBIDX%3AIT
I got this setup for mine and it has tons of stopping power now, Way better than any cable operated caliper and at about the same cost as a good cable operated one, the feel is a lot smoother too. The only thing about this particular one is that it comes pre assembled and bled, and it has a rather long brake line so if doing the front wheel it would need a shorter line or some clever routing , I chose to do some clever routing and zip tied the line to the rest of my cables for a clean looking install. For the price, this one is really hard to beat, the stopping power is awesome, and it's real easy to install.
Since my front end wasn't set up for disc brakes, I got this bracket setup for the caliper off ebay and it works great... http://www.ebay.com/itm/271571281671...%3AMEBIDX%3AIT
, or you can get some of those machined billet brackets that sit in the wheel dropouts which also work really well, especially for the rear wheel.