A national retailer can be just fine, though you can often find really nice guitars used, but it's not recommended to buy one used unless you have played for a long time and can tell if it's been repaired properly, maintained, has neck bow, problems with intonation, curved frets, etc.
I'm going to go ahead and assume you don't care if it has electric pickups. Keep in mind 90% of acoustics in the $700+ range have electronics.
Just to recommend a few off of long & mcquades site since you brought them up:
$999
http://www.long-mcquade.com/products...th_Cutaway.htm
Top of budget, has pickups if you want to plug it in, great action, great tone and fantastic construction. My buddy has this exact model he grabbed last year, it's absolutely rock solid.
$790
http://www.long-mcquade.com/products...Spruce_Koa.htm
Cheaper, a little worse electronically, but the body build is great. Koa fretboards are fun to play on. Martins typically are bit fat in the neck though, so if your hands aren't big it might be a bit much. Always try before you buy of course.
$720
http://www.long-mcquade.com/products...tars/DR_S1.htm
Straight dreadnought, no cutaway. Not great for performing but fun to play at home. Good solid tone, the DR S1 is an old standard. Very deep tone.
$550
http://www.long-mcquade.com/products...e_Sunburst.htm
Yamaha's are vastly underrated, typically having above average build quality and quick action. Good price for a good acoustic.
Now, take my recommendations with a grain of salt, as I personally only own 1 acoustic lead guitar being a bassist by trade, but I've played on many of them. ~800-1000 dollars is a pretty nice sweet spot for acoustics however, which is good considering the sweet spot for electrics is ~1100-1500.