To wrap this up in case others find it difficult finding a 5 to 6 liter cyclinder:

Worthington 40cf (apx 5.6 liters) galvanized steel, fill pressure 3130psi
Length: 18.75 inches
Diameter 5.4 inches
Weight 18.1 pounds
Bouyancy full -6 pounds
Bouyancy empty -3 pounds

It is slightly smaller diameter than the 4 liter Drager cylinder. I have not heard of Worthington before, the shop manager said that he just got them in in the last month.