Maybe I'm missing it in my documentation but can I use any power injector with this Aironet or do I have to use a proprietary device?
You don't have to use a Cisco power injector (although I would :) as long as the type you used met the spec's below;
The Cisco Aironet 1240AG supports local power as well as Power over Ethernet (PoE), either by an IEEE 802.3af-compliant Power Sourcing Equipment (PSE) device, such as a switch, or by a device capable of providing 12.95W to a Powered Device (PD) and compliant with Cisco Inline Power.
A Cisco Aironet power injector can be configured to order, or can be ordered as a separate item (below). If the power injector is configured to your order, it will ship inside the packaging box of the access point. If the power injector is ordered as a separate item, it will ship in a separate box.
The power injector does not come with a power supply. Instead, it uses the power supply from the access point. If you intend to use the power injector, be sure that the power supply will be included with your access point, either by selecting the "AIR-PWR-A" option under the "Power Supply" category, or by taking no action for that category. Do not select "AIR-NO-PWR-SUPPLY" for the "Power Supply" category.
The power injector can be ordered separately. If you previously ordered the access point without a power supply, you will now need to order a spare power supply to use the power injector. See Table below for power supply spare part numbers.
Cisco Aironet 1240AG Series Power Injector Product Numbers
AIR-PWRINJ3 Cisco Aironet power injector for the 1240AG Series
AIR-PWRINJ3= Cisco Aironet power injector for the 1240AG Series (spare)
AIR-PWRINJ-FIB Cisco Aironet power injector media converter
AIR-PWRINJ-FIB= Cisco Aironet power injector media converter (spare)
From this doc;
Hope this helps!