Marie,
The oil filter is common to a lot of mitsubishi cars, and you can pick one up in any motor factors. The standard plugs supplied by mitsubishi are NGK plugs
Spark Plug, 2.0 V6 MIVEC, Front GPX GPX NGK BKR7EKC-N NGK
Spark Plug, 2.0 V6 MIVEC, Rear GPX GPX NGK PFR7M NGK
http://www.fto-ireland.com/parts_listing.html
Those part numbers are NGK part numbers. Front plugs are 'regular' standard life plugs. The rear ones are platinum tipped long life -hence the different part number and higher price! But physically they are all the same. So you can fit the platinum, long life plugs on the front cylinder bank also, as per MfFTO's suggestion.
Note however that you need to remove the inlet manifold to get access to the rear cylinder bank. In addition to this the spark plug 'barrel' on the mivec engine is keyed, that means that you need a special spark plug socket to fit these plugs -a regular spark plug socket will not fit in the barrel. If you are going to contact the mitsi dealer for parts be sure to have the chassis number to hand...
Best of luck