Check CraigsList in your town, and surrounding area. I have found a few things on there, and usually they are within driving distance from you, so you get to see before you buy. Check some of the classified ads on different bike websites, too.
There are no "good" deals on Thunderheaders! They are expensive and the manufacturer makes sure he gets his price for his product. As mentioned above, the heat shields are extra. I've used a Thunderheader on my Low Rider and a V&H ProPipe. Personally they both made good power.