Hi tony,
we use this at work for custom striping jobs, Scotch® Fine Line Striping Tape, 8 Pull Outs, 6314, 1 in x 550 in. its 1 inch wide but split into 8 pieces so u can remove individual strips as required. its ideal for what i want. how ever i believe its astronomically expensive. maybe u could try a local car body shop to see if they have some.
for my own use, i use a pin stripe of the required width & place it on the model in the correct position. then i place another pin stripe either side of that 1 & remove the original stripe, leaving a neatly masked line of the correct width for painting.
masking tape doesn't work well because the paint seeps under it, plastic tape gives a much better edge, especially if u remove the tape before the paint is fully dry
cheers glenn