I'm toying with the idea of running a buried natural gas line out to
the garage for a hot air furnace as well as my foundry furnace (now
propane)

I'de have to cover about 25 feet from the basement to the detached
garage.

What are my options for piping? I recall hearing about a direct burial
stainless product, but cant seem to find any info on it for a DIY
installation.

I relaize cast iron is not suitable for direct burial, but is is OK
within a counduit of some sort?

I'de prefer something flexible for easy of installation, but will
gladly make do with whatever works best.

Vin