The place I buy from online is isuzutruckparts.com ,they have an 800 number 800-338-9249.The guy I deal with is name Josh his direct line is 657-200-7831,very friendly and helpful.Even with shipping it's worth it.
As far as prolonging the life of any vehicle,it boils down to maintenance .Use synthetic oils,long life anti freeze,I use the red stuff,it's expensive,but worth it.Most people don't realize the importance of antifreeze,I didn't either until we blew an engine on a Peterbilt
tractor and the engine failure was attributed to using the regular green anti freeze and not checking it enough,and the water jacket in the engine block corroded and caused premature engine failure.The anti freeze looked fine,but it wasn't.The biggest thing I can say about prolonging the life of a truck is take it easy and and don't drive it hard.This is especially true with these little japanese trucks with automatic tranny's .Driving them hard with a heavy load will kill that tranny and little 4cyl turbo diesel.That's why employee's are the worst thing for any vehicle,they will beat the crap out of your equipment and never respect it like you do,as soon as they are out of your sight,not all,but most.They are not paying for it,especially when it breaks.They will yes you to death to your face,and will beat on it when they are on their own.There are exceptions to this,like if you buy a brand new truck,and hand a good responsible employee the keys,and let him know it is his responsibility ,and he his the only one that will drive it,they will generally take care of it,but not always .GPS in trucks help,but it is not the total solution.Making it a practice to pretrip the truck everyday before they start,check the fluids,put the four ways on and walk around the truck and kick the tires,check lug nuts,look under the truck for broken springs etc is a good practice,and one I adopted driving tractor trailers with 100k lb plus loads over mountain roads,and do with any truck before I start my day.Hope this info helps.