You can buy buluns mostly anywhere they sell tv accessories, but I was just in Radio Shack a couple days ago and bought a new balun for mine, and I would have to say they have what you may call a more heavier duty and waterproof then the one that comes with the scantenna, thats just my opinion.
Also a good alternative to RG-6 and being able to use a heavier cable with less loss and no adapters is RG-11, there are some pretty good prices on ebay for that stuff.
Just my .02...........