Most hardware stores have glass cutters there. They can normally cut any kind of piece of glass for you, but remember the more intricate the cut the more expensive it will be.
Depending on the bowfront and manufacturer, you could probably find one pre-cut for the right tank size and degree of the bow without having to get one done customized.
If it is one of the All-Glass or Marineland bow fronts, then you should find some lids for it at Petco or Petsmart. Shouldn't there be a cover already for that bowfront?