New tires! 90/90-12 (i.e. 12-3.00) from Bikebandit.com arrived. $47 shipped for the two together. They're a slightly tight fit on the rim and take a short while to seat once the tubes are inflated. But it probably would have gone smoother if I had bother to wet the bead of the tire before trying to get them on.
The difference in handling is easily felt. While my speed on level ground is unchanged after recalibrating the bike computer, I now slow down a bit less on hills and the traction improvement is great. No skidding under hard braking, and no sliding on fine gravel.