You can usually find pinstripes at any large chain store. Autozone, PepBoys, heck, even Wal-mart stocks it in some locations. It is much cheaper than the $100 the dealer charges.
The $100 that the dealer charges usually includes installation- which is what most of the money goes for (and profit of course). Most people don't have the talent and patience to do it correctly themselves. (Which results in a ghetto-like finished product)
If you don't have the patience and skill to do it yourself- you could always look for detail shops in your area that will probably cost less than the dealership, while still maintaining a professional look.