You can buy cruel weapons. You can use the 3 chipped/normal/flawless recipe for getting cruel weapons with 2 sockets. You can gamble for them, You can craft or let Charsi imbue items which might get the cruel affix. It's just a prefix which can appear on magical or rare items. Any monster in hell should have a high enough level to drop them.
MF generally increases the chance of getting a cruel item, simply because magical and rare items will be more numerous with high MF. However, it won't increase the chance of a magical or rare item having that affix, i.e. the number of cruel magical items in comparison to non-cruel ones won't be affected.
In that case it might be a good idea to be more clear about MF. It doesn't affect the vendor inventory or improve your luck when gambling. It only has an influence on monster drops. However, a Gheed's Fortune charm reduces vendor prices which affects gambling prices as well.
Gheed's Fortune, a unique Grand Charm, lowers vendor prizes (as well as giving you some MF), and combined with an Edge runeword (very cheap to make), you can get up to 30% off vendor prizes....very very nice when gambling. Or in this case, shopping for cruels.