[Dougal]

Belts and filters you can buy from wherever, there's not much point sticking to genuine for consumables such as those.

Gaskets, stick to genuine. I go straight to the local Isuzu dealer. Their prices are often much sharper than aftermarket crap.

 

[Dougal]

This is the brand I use
http://www.rycofilters.com.au/catalogue/application/make/Isuzu Utilities & Trucks/model/NPR59P

Z155X is the bypass filter
Z333 is the full flow filter

Unfortunately their online cross reference only works to their own filters, not from their filters to other brands.
What brand were you using? Many have online reference charts.

 

[Dougal]

My Z155X (big fat filter) is the bypass.