B&Q definitely have them (but I wouldn't want to carry back to E&C from there). They're out the back with the gardening stuff.
Not sure if Tescos or Asda on OKR have them. They were going to be our next stop if B&Q hadn't had them on Sunday. But they're also a bit of a trek from E&C itself.
If you're Nth side of E&C, then Denise's Florist or the greengrocer (both on Gt Suffolk St) both generally have trees for sale. Denise's had trees when we went past on Sat, but I didn't look at the g'grocers (although we got our tree from there last yr).
If you're Walworth side, wouldn't someone on East St Market be selling trees?