Just really hard to find, and doubly so at a reasonable price. A pair of new-old-stock rims seems to sell for approximately $500US - excessive to say the least.
Paris-Roubaix's are tough-as-nails rims, made to get down and dirty in the spring classics. They're made to be ridden, not hung on a wall and admired, so I was quite happy to find this gently used set, formerly the property of a Dutch rider from the Agu-Abus-Koga-Cordo team. Patience and careful watching let me pick up them for a fraction of the price of NOS - laced to a really nice set of Shimano Dura Ace 7403 hubs. They'll be relaced to a set of Mavic 571/2 hubs sometime between now and April, 2011 - at which point they'll revisit their cobbled past.