Bissell 3750 Upright will not stay in the upright position?

I have a bissell Liftoff Upright vaccum model 3750. The unit will not stay in the upright position. There is a plastic cover that has a lip on the back which locks and allows the unit to stay upright. So when you press the foot pedal in the back it will normally release so you could push the unit around the floor. I called Bissell and requested that they send me the part and they refused. They said I had to bring it to their Authorized reair shop. I know how to fix it; but can not find the piece anywhere. Does anyone know where I could buy this plastic cover?