The 23" Sleeves and tyres can be bought from http://www.pegasushobbies.com/rims.htm
Take your standard Tamiya, Fujimi or Aoshima wheel.
(Tamiya Nissan Skyline wheel shown)
and cut the wheel in half or less using a hand saw etc.
As the rim will not fit on you will need to reduce the outside diameter
of the wheel to fit, I do this by placing the wheel into an electric drill
and filing or turning it down, don't press to hard or you could break the
central shaft off the face.
Keep checking the fit, if it goes too small it's no good at all.
The internal diameter of the Pegasus rims are tapered towards the front
radiused face, so watch which side you are using.
The wheel on the left has been turned down just enough to fit.
the one on the right is standard.
This is showing the finished rim/wheel conversion
it would seem that only 18 inch rims will fit as 17" or lower are too small.
This is the same idea used on Fujimi's Volks Racing 18" TE37
Finally this shows the finished setup fitted to a Tamiya Skyline R34 GTR