You’re looking for this:
I bought mine from Digi-Key. The problem that you’ve got is that your 24VAC is actually RMS. Peak voltage is actually ~34V. Furthermore, if your AC wall-wart is like mine, you’re actually closer to 40V peak. The neat thing about that regulator (besides everything) is this:
[quote]Operation at 40Vdc: Operating with inputs up to 40Vdc is permissible if, for inputs
between 36 and 40Vdc, the maximum load current is reduced to 0.35A for 7805SR-C
and 7803SR-C, and to 0.3A for 7812SR-C. Under no circumstances should the input
voltage be allowed to exceed 45Vdc.[/quote]
This regulator, plus a rectifier and a filtering cap, and you’re good to go.