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A lot of people buy injectors from Warren Diesel and they seem to have good customer service.
That said, there are quite a few threads on various forums where there ended up being problems with their injectors.
The remaned Motorcraft injectors have the best reputation, but there are counterfeit one out there, so choose your source wisely. AutoNation White Bear Lake is a good source with low prices.
It may not be true anymore but when I was researching a couple years ago, Ford was the only company with access to all of the small parts inside the injectors. The other companies had to inspect, measure and re-use some of that stuff. Depending on the supply of cores, the acceptable tolerances could vary a bit from some rebuilders.