Turner's Outdoorsman will usually have the best prices, but it appears (after a quick check of their website) that they don't have any 500s right now with a 28" barrel. Big 5 Sporting Goods is another great place to check in addition to Bass Pro.
Don't be too concerned with the wood vs. synthetic stock. As long as you can get the gun you like at a price you can afford, you can easily upgrade the stock later. Like I said before, I HIGHLY recommend Boyd's Gunstocks - they make a high quality product and the prices are great. You can get a really nice hardwood buttstock and foregrip set for a Mossberg 500 for about $120, and it'll take you about 15 minutes to install. You'll find that, unless you go with the nicer and more expensive shotguns ($500-600 and up), you're going to get a cheap lightweight buttstock whether it's wood or synthetic, so you'll probably find yourself wanting to upgrade down the road anyway.