Ebay seller "Dridiot" sells them. He doesn't have the Derby brand replaceable straight in stock right now, or the already-broken-in-half Derby blades to put in it, but this one: BLACK FILARMONICA Barber Razor - 1/2 Double Edge Blades - eBay (item 260474237668 end time Nov-07-09 08:28:02 PST) should work just as well, but you do have to snap the blades it comes with in half. I think classicshaving.com also sells replacement blades. They sell replaceable blade straights, too, but are quite a bit pricier than the link above.
Thanks for the props, guys, it's still every bit as much fun now as it was a year ago.
And I hear ya about bald arms, legs, etc... I went from looking like a wookie a year ago to a shorn sheep today. :)