I did some quick search and it seems many low-voltage landscape lights are rated 12V (either AC or DC). I believe these can work with 24V AC if you connect two such bulbs in series. Alternatively you can connect a diode (such as 1N4007) in series to block half of the AC waves, thus effectively reducing the average voltage to 12V. Another alternative is to find 12V LED lights — because LED lights are inherently diodes, they should be able to work directly with 24V AC (I would recommend adding a power resistor in series for some added protection).