A couple of free calls to different dealers (having the part # would be helpful) will tell you if your "service" mgr. is blowin smoke or not.
As Dallas said, you should be able to cross reference to another brand...but being warranty, they'll probably insist on genuine Isuzu replacements.

St. Charles Isuzu : 1-800-727-8066 (Merlin)

Wentz Isuzu 1-800-937-9368 (Lee)

Good Luck, Ldub