Here is a link to the place i get all my parts from if you cant find a part just give them a call they always were friendly and helpful when i called.
Performance Nissan KN Cold Air Intake also im not sure on the injen specs as far as pipe diameter but here are the dimensions for the k&N the should be ruffly the same..
Part RU-4960XD Product Specifications
Product Style: Round Tapered Universal Air Filter
Base Outside Diameter: 6 in (152 mm)
Flange Inside Diameter: 2.75 in (70 mm)
Flange Length: 2 in (51 mm)
Flange Type: Centered
Height: 5.5 in (140 mm)
Top Outside Diameter: 5 in (127 mm)
Top Style: Rubber
Top Material/Finish: None
Filter Material: Cotton Gauze
Filter Re-Oiling Amount: 1.43 oz (42 ml)
Weight: 1.2 lb (0.5 kg)
Product Box Length: 6.88 in (175 mm)
Product Box Width: 6.88 in (175 mm)
Product Box Height: 7.88 in (200 mm)