Driftwood or bogwood? It's just a case of keeping an eye out in your lfs and jumping on a piece when they have it in store. Keep a look out on ebay etc too.
Go down to your local lake and you'll usually find a load of drift wood washed up on shore.
I picked up a lot of bog wood from my granda's land in Donegal last weekend. Straight out of the ground. Obviously you should soak and clean anything you find.