well, if you buy it in the stores, you'll quickly find that they usually rip you off as far as price on it. you can definately go and get one off the beach, but you'd have to do a bunch of stuff to it...i think i saw where someone said that you should clean it w/a very mild bleach solution, but dont do that yet...wait for someone else to approve of that, as i'm not 100% sure on that...and i know for sure that you should boil it...get as big of pot as you can, and if it fits in it, boil it (not sure how long tho) or if it wont fit, boil water, and dump it in a rubbermaid tub or somethin w/the wood in it. you'll probably find that while its soaking in the hot water, it will turn the water very brown. Nothing to worry about, as its just the tannins coming out of the wood...kinda like how tea works, this will just stain the water that color. But if you continually empty and refill the water that its soakin in, it should go away after a while...i still get a little bit out of mine after about a year, but its to a point where its only getting dark about 1 or 2 times a month, right when i do my water changes, then its back to clear. You could also run carbon in your filter, and that makes the water really clean.
also, you might have trouble getting it to sink...only time (and hot water i think) can make it sink, unless you're really impatiant (like i was) and use something to weight it down..i had mine floating in water in a rubbermaid out in the arizona sun for the whole summer, and it still wasnt sinking completely. I had also been putting a brick on it to sink it...well i got impatient and decided to just tie a baggie full of sand to it, which was all that it needed.
hope this helps