This is a really old piece of code, but this page still gets hits for some reason, so I've updated it, and allowed it to continue to exist.
UPC-A numbers are 11 digits in length followed by a single check-digit (1-5-5-1). They are a subset of EAN-13. Do not enter the twelfth digit, as the generator calculates the check-digit automatically.
Enter the code, hit 'generate', and you'll get it. If you want, click on 'range,' and it'll output (21 at a time) a sheet of them, incrementally.