I get my seeds from Johnny's seeds, Home Depot and or Ace Hardware.
Johnny's is the most reliable.
I have had great luck with most kinds of lettuce, kale, chard, bok choi and tatsoi.
Also good luck with peas, squash, mellons, chillis, tomatoes. However I plant these plants on the edge of the GrowBot and get a simple trellis to help support them.