You can push 24 VAC over 400 meters, as long as your power supply accommodates for the voltage drop.
1. Find Camera Amperage
First, you need to establish how much current your camera pulls during operation. This can be determined from the listed wattage on the spec sheet:
I = P(W) / (PF × V )
Where I = Amps,P(W) = Watts, PF = Power Factor (We'll assume it's 1), and V = volts. Therefore:
.291 A = 7 /(1 x 24)
The example above assumes a camera is listed at 7 W, yours will differ. It pulls about 0.3A.
2. Determine Wire Resistance
From there, we can use this formula to determine 'Voltage Drop' due to the resistance of the wire and distance of run:
Ohms x (Wire length/1000) x Current = Voltage Drop
Luckily, 'Wire Resistance' is a standardized value for copper cable, and those factors are commonly provided by cable manufacturers. Here's a table of the most common gages used in Low Voltage work:
3. Calculate Voltage Drop
So for our example, we'll use 18 gage wire over 1350 feet (~400m):
1350 feet of 18 gage wire with a camera that draws 0.3A plugs into this formula:
Voltage Drop = (Wire Resistance) / ((Distance)/(1000)) x (camera amps)
2.6 Volts Dropped = 6.51 x (1350/1000) x .3 Amps
This means your power supply needs to nominally deliver ~+15% over 24VAC to power the camera
4. Specify the right Unit
Luckily, most supplies are rated to deliver a voltage within a range, and some are built with this loss in mind. Take for example this Altronix unit:
This particular unit will perform the task of our example, with voltage drop and amperage considered. It costs about $120 USD online. However, like I noted up top, your camera's amperage draw makes a big difference.
So making a long story short: No 'special' power supply is needed - just one that's properly specified!