You really just have to watch the sales. I know sometimes toys r us has great sales. I've purchased some from there. If you buy the big pack of three you can get a single for free. Or spend 25$ you get 5$ off.
I have found a couple of cheaper ones at Walmart in Sherwood. The best place that we have found them is used at Game XChange in Lakewood Village in North Little Rock. We check there one every couple of weeks and normally find one or two that we do not already have.
I actually got my brothers whole game set off of Fingerhut, he really wanted the newest game "Swap Force" for Christmas and I was able to buy that and do it in a few payments instead of one large payment