The LBS is definitely your friend. Lots of them sell used bikes along with new bikes, and sometimes you can find a real bargain on something that's in great shape with lots of good stuff already on it.
I recently bought a seat, seat post, and neck at one shop around here, and then I asked about an under-the-seat bag. I looked at one that had the shop's name embroidered on the side, and said, "I'm guessing you guys like this one."
The guy said, "we do," and gave me the $15 bag for free. Nice, it was just like getting a 10% discount for nothing.
I've found that most bike store guys are really cool and very knowledgeable.
They'll also help make sure you get a bike that fits you.