I picked up a kit at an Isuzu dealer that included the gasket and filter (a few years ago and it's not longer a dealer, but they still offer service and have parts) - the gasket was flattened against cardboard with shrinkwrap to keep it so. It wasn't a lot of money - there's also a copper crush washer or two that you can replace (don't have to but in the CD manual it specifies to) - they gave them to me for free for buying the kit....

-- John