Couple ways you can replace them..  1st is to contact the manufacture (if yours is still exists) and ask them for replacement decals, usually the easiest but more expensive then other options.  2nd - would be to look on Ebay, quite often you'll find dealers selling off complete NOS sets or even partial sets, most times this is the cheapest way to go and 3rd would be to either take pictures of your existing decals or find good pictures of them on the web and take to a vinyl sign shop and have them duplicate the images, depending on where you go and options with regards to the manufacturing of the decals it can be either expensive or fairly cheap..    Sometimes you'll find the shops selling similar decals on Ebay and a quick email to them and they will produce the decals wanted for a decent price and then they will start selling them as well...