Central Sod Farms
http://www.centralsodmd.com

Just over on the other side of the Bay Bridge, past Annapolis.

PS: If you do end up buying from Central Sod Farms, please let them know that one of the guys from the rocket club recommended them. The farm lets our club use their fields to launch model rockets and has been for several years. Thats how I knew about them. Thanks!

I wouldn't expect any seed planted between December and now to come up. The soil (despite the mild winter) is way too cool for efficient germination. Spring seeding is not a healthy gamble around here. You definitely want to put down a layer of compost (NutraGreen is a good one) before sodding. You should till it into the existing clay. Grass will grow fine in Northern Va. clay as long as it isn't excessively rocky. If it is Manassas (Culpeper basin) red clay, it will grow fine.

Below are two places I recommend:

Chantilly Turf Farms
http://www.chantillyturffarms.com/

Great people.

Also, Woodward Turf Farms out in Remington is good.

http://www.woodwardturf.com/

Both are pretty convenient to Northern VA.

The most important thing is that you water to the point of excess. April 1 is probably the ideal time to put the sod down as you want to avoid freezing temps yet have enough time (6 weeks or so) to get it established before the heat of summer gets here. Best of luck to you!

Not sure why this isn't posting, but here I go one last time: I am not sure what your watering capability is, but keep in mind that sod has to be saturated with water for 3-4 weeks after installation. This usually means daily watering of at least 2-3 hours. If you can't do this, sod is not the best option.

Thanks to those who responded. Based largely on the recommendation of a local landscape designer, I ended up using Settles Turf Farm in Annandale. Darius Bly offered an attractive price, provided the installation and, as it turned out, a great result.