A bag of any of the commercial substrates should work fine. I'd get one that contains nutrients since you want a carpeting plant. If you go with Flourite (my own favorite), be sure to rinse it all really, really well before putting it in your tank.
Dwarf Sagittaria will probably be your best bet if you're looking for a carpet that can fill in fast without needing CO2.
Dwarf Sagittaria will probably be your best bet if you're looking for a carpet that can fill in fast without needing CO2.