I am tossing the idea back and forth for a fall/winter garden myself.
My poor loose leaf lettuce died in the heat.
The tomatoes are doing well, but the squash which had started out
lovely is suffering in the heat.

For a fall crop I had thought of cooler loving plants such as
spinach , loose leaf lettuce or even brussel sprouts.

I think if I start in late August beginning of September the plants
should be fine as far as heat.
And supossedely we will be off the water restrictions by the 1st of
September.

As far as online seeds I have had good luck with them. I have used two
companies.
Thompson & (I can't remember the second name) but they have a huge
variety of seeds.

And Heirloom Seeds they have very interesting seeds avialable.

good luck,
Anita