I think Keyspan is no more, but they are available on eBay

Tripp-Lite seem to "own" the Keyspan range these days at more than silly prices (didn't pay that much when I bought mine new)

$139.99 http://www.tripplite.com/en/products/model.cfm?txtSeriesID=849&txtModelID=3915

$29.95 http://www.ebay.com.au/itm/Apple-Mac-USA28X-Keyspan-USB-Twin-RS-432-Serial-Adapter-/150623524866?pt=LH_DefaultDomain_0&hash=item2311dc9802

Looks like Keyspan serial/usb adapters are now in the same realm as Griffin iMates. If you see one for nix, grab it. Someone here will want/need it.