We do T-giving at WDW every year--it's our family tradition. This year we'll be at BCV.
We've tried lots of WDW restaurants for our T-giving meal--I like the traditional meal, but my DS prefers good steak. So we keep looking for the best of both.
Here's what we've found so far:
California Grill is an excellent meal, but you can't get enough "traditional" stuff for it to qualify as T-giving for me...the Garden Grill at The Land in Epcot is superb...and has both steak and turkey (and catfish) as well as sides in its family-style meal. It's our favorite.
The steak houses serve really good steak (DS loves it), but not traditional T-giving offerings--one year, in fact, we booked at Shula's at the Dolphin after I'd called to see if they would have turkey...they said yes, but when we got there, they didn't. The rest of the family loved their steaks (as did I, but I was so busy being disappointed at no turkey it took the edge off for me).
That was the year I discovered that the counter service "special of the day" at SSR's Artist's Palette served the best deal in WDW...a very good traditional T-giving turkey-and-stuffing-and all the required sides meal, for $10--including pumpkin pie, rolls, green beans, cranberry sauce, et. al. Now, I indulge my DS and let him get his steak mid-day, and then we head over to SSR for "real turkey" for dinner!
We don't cook (at home, much less at WDW) so can't comment on grocery-store options. But can tell you that T-giving restaurant reservations fill up very fast, very early, regardless of the type of food. So do make yourselves an ADR, even if you wind up deciding to cancel it in favor of an in-villa meal.
Happy T-giving!