The one at Radio Shack is cheap, but it does not have white noise, only brooks and other noises that vary and which I find irritating. They used to have one that had white noise, but it only operated on batteries and had no A/C adapter and would only last about 3 hours. Sadly, the best one comes from Sharper Image and it will run thru a 4-pack of double A's in about 30 seconds. I have taken mine back twice and it's just the way they are made.
The thing that totally pisses me off about the Sharper Image one is that I don't even use it on battery power. Not as a sound machine. I unplug it and put it in a suitcase and by the time I get where I am going it has run thru the batteries and is dead. I then have to reset the clock and the alarm and the F or C choice, etc. When it's not a sound machine it's just a ******* clock and a clock ought to go about 12 years on a set of 4 AA batteries.
We're using a DEK one for our 5-month old. Works really well and plugs in. No white noise, but 8 other sounds, including one called womb. We just leave it on ocean sound and Isabelle likes it. $20 at babies r us.