There are countless places to buy online from, but shipping will kill you (if you're cheap like me) and in most cases, you do not get choose your piece (definite deal breaker for me). But if you have the budget, there are many places like manzanita-driftwood.com where you can buy specific pieces.
As far as manzanita vs spiderwood, that depends on what look you're going for. Spiderwood usually isn't very big, so it might not fill in your tank as well as some other species.
If your LFS has a piece you like, but is on slate, you can always remove it from the slate. Just be aware that most driftwood will float if not attached to something until it is fully waterlogged, which can take days to months to never. One that never fails to sink is mopani. Any interest in that?
Another option is finding your own driftwood. Local creeks and lakes can be a goldmine. I'd stick with hardwoods and make sure they are seasoned and properly cleaned.