There are several versions of 24" but the 24" that is on the front wheel of a terry bike or on an airnimal folding bike is called 520. These guys sell very nice wheelsets, and I think will split them up for you if you just need a front one.

http://www.calhouncycle.com/productc...idCategory=114

Another possiblity is posting at the terry website in the for sale section.

Junior bikes are also built with 24" wheels, but I think they use a diff size, maybe 540, like wheelchair wheels, so be sure to get what you need. High performance tires for the 520 size are also available from calhoun (I like the schwalbe stelvio on my terry, and I also like the velocity uriel wheels).