You can go to the local Bosch dealer. Probably a lot cheaper then the Ford dealer! (Here in the Netherlands it's a big difference in price)

Bigger ones aren't a solution to anything. Most of the Jetronic injectors have the same flow rate. For example, the 28i V6 engines use the same injectors as the XR3i. They only have a different length but specifications are the same.

The injectors are good for big BHP figures but the rest of the Jetronic system holds you back at some point. That's what I learn't in the past and I will be using standard ERST injectors in my car when it's finished.