When used as a noun, they are essentially the same... though 'stock' implies goods that are already present, while 'supply' can imply a steady flow of goods. When used as a verb, 'stock' is the act of putting goods into place while 'supply' means to give those goods out. So, in your example, the "stock of housing" is how much 'housing' that currently is present in rural areas, where "supply of housing" can mean the same or can mean the amount of housing that can be made available. In your second example, both words are the same and both are fine.