I don't know for sure, but I would hazzard a guess to say there are only a couple manufacturers who make torsion bars so they are all coming from those couple of sources.
About 15 years ago when MP reproduction bars came out I bought a set from Mancini for $250. In the ensuing years they have slowly dropped in price to the $130 range at MP dealers. Others like PST and Just Suspension were still charging $250-300 a set. I think MP simply saw an opportunity for more margin, changed dealer prices and viola! everyone is charging $300 a set for new bars.
Yes, there are still dealers out there willing to sell old stock at the old price. I've also noticed that the 1.14, 1.16 and 1.22, which are less popular, are also still available around $130 a set.