Mate,

Just ring up any sign graphic suppliers... Supply One, AGS etc and ask for Clear Application tape 610mm wide in low or low to medium tack.
There are 2 types, 1 is flat clear.. the other textured.
The clear is easier to mask out from as you can see straight through it, it tends to curl when the paint is drying though.
Textured is very slightly opaque ( but still very good visibilty), this doesnt curl as paint dries.
I use the flat clear a lot more than the textured