Do they need the 547mm, 540mm, 520mm, or 507mm rims that are all labeled 24"? I will guess the 540mm size rim which is the same as most wheelchairs. Also called the 600A size. Also called 24" x 1 3/8". Or you could go with the 520m aka 24" x 1"

Both tyres can be found here:
http://www.everybicycletire.com/Shopping/c-17-24.aspx

The 520mm rim: http://www.everybicycletire.com/Shop...ohead-rim.aspx

520mm wheelset: http://www.everybicycletire.com/Shop...elset-520.aspx

If you build your own wheels, the Aerohead rim sold separately will be the "racier" one. Plus you can get the hubs of your choice.