You can definitely find either a drop-in or undermount silgranit single bowl sink that will fit a 30" cabinet.

If you have the flexibility to play with the width of your sink cabinet, you have to determine how big a sink you want, get the space requirements from the company (Blanco and Franke have good sinks and they have great technical specs on their websites), and then ensure that the INTERIOR of your cabinet will accommodate those measurements. Keep in mind that, typically, undermount sinks require larger cabinets than identically-sized drop-in sinks - all things being equal.

There are a lot of single-bowl sinks out there that are under 30 inches wide. But I have noticed that there seems to be a bit of a break between the low 20-inch range and over 30 inches, if you are looking at a single bowl in that size. Blanco has a silgranit single bowl at 22" interior width. But the next size up single bowl width is right around 30 inches - too big for your cabinet.

Do you really need a super-sized single bowl? I pulled out the tape measure and realized just how big a single sink is that is 22 inches interior width - holds a cookie sheet with 5 inches left over! Of course, you are the judge on what you need, but I surprised myself when I did the old tape measure test.