Alex, if you are going to use a basin for them to dig into, you dont have to use a Sand/Loam mix, the Sand is generally used for Farms, because it shows the ants up better inside tunnels, but as you wont see any tunnels, this isnt an issue, heres an idea for you that will work just as well...
Sand/Peat Mix - 5 parts Sand, 1 Peat (ie if you put 1 cup of peat, u need 5 cups of sand)
The reason you use a Loam mix, is to get the sand sticking together, but Peat will work just aswell.
You need to prepere the Peat, u can buy a bag for about £1 from virtually anywhere, make sure it doesnt have any pesticides in tho, it will say on the labels if it does.
You need to grind the Peat up, u will notice when u open it, its very chunky and full of bits, the best way to do this, is using a Food Blender, u'll probly have to convince your parents that this ok to do, if they are anything like myne lol otherwise, u can do it using a cheap flour sieve, to filter out the big bits.........basiclly the idea is to get hte peat you will use for mixing as fine as possible
Once you've got your peat nice and fine, mix it with the sand.
Now, very important! Preheat your oven to 200C, you need to cook the sand/peat mix for 30 minutes, this will kill any nasty bugs
Once thats kooled, add your clay balls, add your new sand/peat mix, then add sum water.....hey presto

a perfect substrate for your ants to nest in
hope that helps

u dont need to buy expensive sand/loam mixes