Well i've never used a soil substrate in an aquarium but if you are after top soil i know you can get it in any garden centre, B&Q etc. for £3 - 4 a bag (should be plenty for a 200 litre tank). Something else i've wondered about but never tried in an aquarium is the bags of aquatic soil/loam sold in garden centres and fish shops for potting plants in garden ponds. Again this stuff is fairly cheap for the amount you would need in a tank.

cheers
neil