With the very large carbonate content of the water you would need high levels of acid to bring the pH down and this would mean equally large amounts of CO2.
This is irrelevant anyway, as the plants need the CO2 not the pH change and CO2 content doesn't change with pH, but pH changes with CO2 content. There are very few plants species which need a less than neutral pH, most people just use plain tap water.
If you want sand, you don't need to go to a aquatic shop. You can use play sand as mentioned, from Argos or the Early Learning Centre, or silver/silica sand from the garden section of B&Q/Homebase etc. There are a variety of sands available in different colours for aquatic use in the trade, but they're not the only ones available.