Ferrite toroid core data
Manufacturer: Magnetics Inc. Tel. (412) 282-8282
Part Number: P-41306-TC
Outer diameter: 0.5 inches
Inner diameter: 0.312 inches
Height: 0.25 inches
Material: Magnetics Inc. P material, 2500u
Al (mH/1000turns) +/- 25% : 1,485
Weight: 2.4 g
Le: 3.12 cm
Ae: 0.146 cm^2
Ve: 0.457 cm^3
Window area: 0.493 cm^2
WaAc: 0.072 cm^4
Winding factor at 60% fill: 0.129 ft/turn
Most any old core will do. I've also used powered iron toroids, but their low inductance factor causes a reduction in performance in the medium wave broadcast band and below.
You could use Fair-Rite #73 or Philips 3C85 instead of Mag Inc P-material. For higher frequencies use Fair-Rite #43 material.