It's not the big and heavy part that matters. It's the bulky part. When you order driftwood, specifically manzanita, you order a large piece that, because of its dimensions, requires a *really* large box. And the people that sell and order manzanita in specific environments, like this one, are interested in a very specific look. So each piece is sold and shipped with very specific dimensions in mind.
That being said, I've spent the past five minutes looking through my bookmarks for a certain art/hobby site that I know I have saved somewhere, which sells manzanita (general sizes, but not specific pieces, if I remember correctly) with no success. But I'll keep looking unless somebody else links it first.
There are also aquarium sites which sell driftwood, and have flat rate shipping. Generally these sites are overpriced, but are very useful when you want to purchase bulky and/or heavy items that aren't available locally.
It's expensive. But the fact that you're looking for a very specific type of wood, grown in a fairly small sector of the US, says that you're willing to spend some money for something tailored to your needs. There's no use in complaining. You could have asked for links to reasonably-priced manzanita and avoided some irritated replies.