The easiest way to do it is on a repositioning cruise where at the end of the season the cruise lines take their ships from one market to another. The positives are a cheap cruise, the negatives are that you're right at the end and beginning of ideal seasons so your experience at each end might not be great. Also you'll have a lot more sea days and fewer port days which could be in either column.

Weather at sea can be considerably worse than coastal weather. Cruise ships are pretty stable and it's unlikely to be awful, but plenty of people do get seasick. I used to be a professional sailor and so obviously can't see what the fuss is about, but probably half the passengers I spoke to felt ill on at least one of the days, and a few people spent the entire ocean transit in bed. If you're prone to motion sickness take some medication with you.

The atmosphere is different to regular cruises. Typically less of a party and the clientele skew towards older and more independent travellers.

To answer your actual question - go to cruisemapper and seascanner and you'll find them easily enough. They're all over the world.

As the other said I was looking for repositioning cruises like 2 months in advance, from one continent to the other. Like I did Europe -> South America 550 euros Costa Cruise last year and Miami -> England NCL 650 euros early this year

The difficulty is finding good prices for a solo cabin. For a cabin of 2 it's even easier to find good prices for repositioning cruises

As the other said, there are more days at sea (less stopovers), and it can happen that some stopover port get cancelled (for example they cancelled my stop in the Azores because they didn't want to bother entering a big Atlantic storm, it was risky)

Same and always wanted to be completely offline for that length of time.