At a good LFS or on many online dealers you can find either Swahala wood or Malaysian driftwood. Both of these will sink on their own and are safe for aquarium use. I personally think Malaysian driftwood looks better, since the Swahala is often lacking in character.
A lot of people on this site use Manzanita. I guess you have to soak it for a while, but it looks cool.
Aquatic Design Group sells Old Black Wood (an Amano name). It looks really nice, but it's probably more pricey.
I'm sure that an internet search would turn up many places online (AquariumPlants.com, floridadriftwood.com, azgardens.com).
And finally, many folks just go out and get their own wood. Pretty much anything you use will leach tanins, but time, water changes, and/or activated carbon will take care of that.