I have not seen a Costco with Propane but have not looked for it...
Wouldn't be a problem putting 100 gallon tanks in the trailer... I do think I would run afoul of some regulation... a friend had a bunch of 5 gallon tanks to exchange and was told he had too many and would need placard... or something like that.
Costco's here commonly have propane. I know BC does too, and I believe Washington State, at least up by the border. Dirt cheap compared to most suppliers.
I imagine there's no shortage of regs in Cali.... my rule of thumb with propane is stay out of confined spaces - underground garages, etc....
Not encouraging lawlessness
Rgds, D.