i buy mine from the local garden centre, its play sand 25kg bag for £1.99, it took 3 bags to do my 5' tank but its quite a thick layer, 1.5" deep. much cheaper than aquatic sand & just as good if not better! i have it in all my tanks.
the best & cleanest way is to empty the tank, keep as much water ,at least half if you can, then clean the sand in a bucket with the tap running constantly in to it, till it runs clear, then put it in your tank & re fill it!
pour the water back in to a bowl placed in the middle of the tank so it doesn't disturb the sand too much, it will still be cloudy, but with your filters back on it will be crystal clear by morning!
I changed from gravel to sand in my 6ft tank without removing the water or the fish. I used a wide hose and siphoned the gravel out via a cullinder with a fine sponge inside, this was placed over a bucket. the gravel remained above the cullinder while the waterdropped into the bucket below and the sponge collected the ***, when the bucket was full I poured the water back into the tank each time.
when adding the sand I washed it well first, and then used some drain pipe. positioned the pipe in the direction of where i wanted the sand to go, and then I just dropped the sand down the centre of the pipe. It was slightly murky for an hour after, but it was such an easy way to change over.