Stacy has the right idea about bribing a metal shop with brownies or beer. A proper shear will leave a clean straight edge and it will take longer to carry the metal to the shear than make the cut. Anyone that works with sheet metal should have the tools to do it...HVAC shops, roofers, auto body shops.
For a fast cheap DIY method that has not been mentioned, use a cutting wheel on an angle grinder. You can use a thin piece of wood clamped in place as a straight edge to guide the wheel.
For a fast cheap DIY method that has not been mentioned, use a cutting wheel on an angle grinder. You can use a thin piece of wood clamped in place as a straight edge to guide the wheel.