Nick -
Glad that you finally joined the site, welcome! Zulu Lima is a member who put together a pretty good map a while back of spotting locations around DFW, it is linked in this thread:
viewtopic.php?f=9&t=33636 departures can be had just like 18 arrivals at Founders Plaza, however, most planes are usually far too high for anything but belly shots by the time they sail past Founders, even the heavies. Your best bet is to stake out a spot in the parking lot near Lufthansa Cargo on the west side, and get on the hood of your car for the departures. Hopefully some of the spots near the fence are open, for a while they had armored cars parked there to prevent spotters from parking. I haven't been there in a while so I'm not sure how DFW DPS responds now, usually they would either say hello and let you stay, or politely tell you to move along, depending on the officer.
The west side is prime in the afternoon for departing photos though, with great light at your back. Hope this helps!