Underground feeder to power shed?

[randomq]

I need to run power to a shed 75 ft away, sizing for 20 amps maybe 30 surge.

I'm thinking of going with 10/3 direct burial cable in metal conduit buried 8 inches or so in ripped pahoehoe and backfilled with cinder and gravel.

Two questions: Is this the right approach? Where can I get the cheapest cable and conduit?

All suggestions welcomed.