Hi and
to Purrs!
I live in a flat too and keep my cats indoors. They have no outdoor access at all and are happy, imo. Opinons vary on this, but a lot of us do keep our cats indoors for various reasons. Yes, they would like to have a garden to play in, but that doesnt mean they can't be content indoors.
There are pretty much 2 ways to go about getting an indoor cat. The first is to look for a cat(s) that need to be kept indoors, eg a cat with FIV, a blind cat, an older cat that has never been outside before etc. Such cats don't necessarily need much additional care and won't necessarily need lots of expensive vet treatment, but they are often best kept indoors. Most rescues will have some cats that are looking for indoor homes and would be delighted at the offer of an indoor home. The second way is to get kittens on the basis that they won't miss what they never had. I have had my cats since they were kittens (one is almost 11 years old and the other is 2 years old) and they have never known the outdoors.
You do need to make an extra effort to provide toys and games for indoor cats. You have to compensate for the fact that an indoor environment is not a stimulating as an outdoor one, but it can be done.
Good luck with finding a suitable cat or cats. I'm sure there is a cat somewhere that's purrfect for you.