I've been buying my Joe Gibbs Racing DT40 from Summit Racing in MacDonough GA.
Once you are on their mailing list I think they'll occasionally shoot you a $20 off coupon for orders over $100. They also discount the price when you buy oil by the case. Comes to around $10 a quart (or less) that way. Also free shipping via UPS ground to anywhere in the US, but I prefer to drive down to Summit with my Boxster - makes for a nice drive (any excuse for a drive is good when you have a Boxster).