Purpose-built “solar hot water tanks” can be very expensive. However, most installations use a residential domestic water heater. An electric version is preferred as it has existing top and bottom heater element ports that can be made into solar ports. For example you can buy a regular electric water heater at most hardware stores for under $500. By removing the heater elements you can easily adapt the 1″ heater port to 1/2″ fitting for your solar water hot input and cold draw. For under 25$ in parts you can convert any electric hot water tank to a solar pr-heat tank.