We got these kits for $10 each from Coventry's (in WA). Each kit does 2 injectors (4xO-rings,2xWashers,2xFilters,2xNozzles), so we bought 4 kits for the 5.0L.
You usually have to get these ordered in cos they are a specific part. I'm sure Ripco can also order these in.
The main reason for doing this was to replace the O-rings because there was an air leak in some of the injectors, I'm not sure how much changing the filter helps.