Water and sewage tanks required

Hi, Can anyone help me in the right direction for the supply of a water storage tank or two tanks giving

me around 15,000 litres capacity? I wish to install below ground.

Also I need a septic tank, I think a three chamber type big enough to service a four bedroomed house

although generally only occupied by two people it should be big enough for 6-8 occupants in case I sell 

in the future. The tank can be over ground or under ground because I have siting options with this.

Thanks