HarleyK's advice is sound. Collecting good driftwood is fun, but can be difficult depending upon where you are at. If you are heading out to public lands you might want to check your state's laws. Here is Wisconsin collect driftwood from public waterways is now illegal. Since all navigable waterways are considered public, the collection of driftwood here is all but over. Chances of getting caught... or getting anything other than a warning... close to nil... but better to know what you are getting into.
Most collected driftwood does nto sink, so you might need to screw a piece of slate onto the bottom of a very large piece. Personally, I do not like the slates. If you do that, clear all the gravel off the bottom, set the slate flat on glas, and then cover the slate with gravel. If there is a layer of gravel under the slate you risk creating an area of anaerobic decomposition of wastes which can release hydrogen sulfides and cause problems in the tank.
I use Malaysian driftwood.