Power supply for LED display
Without directing you to a specific supply,  you already know most of what you need to find. 

Outdoor rated supply, so you want one with an IP rating of 66, 67 or 68.

Constant Voltage, as you need variable output.

Since you need 24v at 8 Amps (195 Watts), adding in a margin of safety, look for a 210 Watt or higher supply.

If you are willing to add in a switching voltage regulator, you could look for a higher voltage supply and regulate down. 48V 5A for example.