Hi Alison,
The Mercadona in Los Alcazares sell a range of Soja products e.g milk, yogurts, also the Mercadona in San Javier.
Eroski at Dos Mares sell Soja cheese they stay open tell 10:00 pm
The only thing I still have not found is a Soja spread or bread.
The supermarket on Terrazas do not sell a range of these products.
Enjoy your stay at Easter.
Hope this helps.
Eve & Clive