24" bicycle tires is such a minefield of different sizes that you need to be VERY specific as to size, not just guessing. There are 4 reasonably common sizes that all get called 24". They are ISO 507 (mtn), ISO 520 (currently common for 24" roadies), ISO 540 (used to be common for roadies but still around) and ISO 547 (rare). Then it may not be 24" at all and it may be ISO 571 (650c). If it realy is 650a then good luck there.
Here's another sheldon brown article which will describe the breadth of the issue,
http://sheldonbrown.com/tire-sizing.html
Regards, Anthony