I would go to your local rescues first. You may find a couple of cats that have lived together and are used to staying indoors. One kitten on its own all day would go do lally - much better to have two but I would still be slightly wary of leaving two very little kittens on their own all day until they were at least six months (only a personal opinion).

Having said all that I absolutely adore the Russian Blue or the British Blue - went for a blue cat in a litter of moggies that were born near to where I live, because I had always wanted a blue cat. Anyway my little Alfie although born blue turned black! and he is the best cat in the whole world (most of the time).