First, soapstone and granite are very different things. Secondly, there should be plenty of 2cm on the east coast. Thirdly, no, I don't think it's common to have soapstone in 3cm. You will find lots of 2cm stones, but not in exotics or more expensive granites (for the most part). It's very uncommon to use 2cm in residential on the east coast. I certainly would recommend 3cm. Why are you thinking 2cm instead of 3cm? Lastly, if you are just using it for a small piece somewhere, you can have it planed down to a 2cm width.

We want to replace our countertop (+/- 30 sf).

We cannot have more than 2cm without replacing our vintage cabinets which do not want to do.

Our preference is Black Galaxy. We cannot find it in 2cm or any other granite for that matter.

How much would it be psf planned down?

You can't have that much planed down. And everywhere should have a 2cm Black Galaxy. MSI is a huge distributor with locations all over the country. I know for a fact that they carry 2cm Black Galaxy at their MD location, and I'm guessing they do at other locations as well. Find a good fabricator on houzz for your area and then tell them what you want. If they can't find it, then find a different fabricator, because 2cm Black Galaxy is not hard to find.

http://www.msistone.com/