The Akrapovič part number for the exhaust springs are P-S1/2. Give Calsport on a call 0845 601 0986 quoting the part number and they will be able to supply you with what you need. They are only a few pounds each.
I know mate, I bought the system second hand and it only came with 8 springs, needed 16 so temporarily used lock wire to hold in place, just had the springs on the headers. I've ordered some up now buddy
I have noticed that a lot of race bikes have both the springs and are lockwired as well, I have done some of mine, I found it really tricky to get to the ones underneath the engine, maybe should have done it before I fitted the system.