4 plugs give ohm reading but front and back two don’t

[IDIBRONCO]

The three best places to buy them from are R&D IDI Performance, Conestoga Diesel (member Agnem on here), or Russ (member Typ4 on here) in no particular order. The prices aren't ebay cheap, but the quality is great from all three. Usually the pump and injectors have about the same service life, if you don't know the history (mileage) of your pump, you should plan to replace that too for the best results.