Single baskets generally are more difficult to master and most people here use a double basket all the time and just stop the shot early - so you get the same volume of espresso from a greater volume of coffee and normally end up with a richer (but not stronger) cup. You can read more about the problem with single baskets at this thread http://coffeesnobs.com.au/YaBB.pl?num=1169781510
You can buy a range of filter baskets from site sponsor Coffeeparts; alternatively we do carry just the Synesso brand single basket at our web store www.coffeetamper.com.au/shop/ as well as a few doubles. The Synesso is one of the better single baskets out there but a double is still a far more regular shape and an easier beast to master.
Hope this helps,