1997 Ranger A/C Work

At the end of may 2002, I had the approx. $100 a/c service at VIOC done on my wife's 1997 ranger pickup. From what I can see a year later it was a waste of money as it did blow out cold air once again that summer, but this year is once again empty. I think VIOC missed finding a leak or something. They sell R-134a retrofit kits at Wal-Mart for between $25 and $30. Or should I take it to the dealer or independent shop to have it fixed?