Bill,
For straight lines the old 3M fine line green tape is the best.  If you have to go around corners then you want to use the 3M fine line blue tape.  Prices have gone up but there are several placed online where you can get it at a resonable price.  Try going to airbrushdepot.com and look under auto body supplies.  I just ordered some and got it for less than $10 per roll depending on width.  You can also go to finessepinstripping and check out their line of tape.  It is thinner than fineline and I really likeit for straight line work.  It will go around bends but not as well as 3M blue fine line.
Alan Resinger