My wife orders a lot of prints online from Shutterfly. We normally get the 8x11 (or whatever that standard size is), but we have gotten a couple the size you're looking for. We've always been pleased with the quality, and the turn-around time is very fast (4 or 5 days to our door).
Sign up (even better, sign up under a few different accounts) for their mailing list and you'll get all sorts of offers. Also, some places will give out free gift cards with purchases (Best Buy, Hallmark, etc, just google who's got what going on). There are also plenty of coupon codes if you google around, and sometimes you can use that in conjunction with the free gift card. We have free gift cards for Shutterfly more often than we don't have free gift cards for Shutterfly. They're usually in the $20 range, with a $20 minimum (so you just pay shipping, basically). We also get e-mails with discounts for things like calendars, mugs, magnets, etc. We don't use most of them, but every Christmas my wife makes personalized calendars with pictures of the kids for the grandparents, so they come in handy.
Their customer service is great, too. When we got married, we ordered our wedding photos and the corner of the thick envelope got bent in the mail, so a couple of our larger prints got bent. We called them that day, and they overnighted (for free) replacement prints at no cost to us.
We occasionally use CVS (order online, pick up in store) when we're in a pinch, but the quality doesn't seem nearly as good.