Wow so...
You've been told by a shelter that you aren't suitable to offer a cat a suitable home for a very valid reason, you have no outside space other than a balcony.
Despite them having vastly more experience with cats than you, you haven't considered for a second they might be right and go straight to Gumtree to buy a cat from someone who either won't check or won't care where you live.
You think your cat needs company, I would put money on it the last thing your current cat, who has no outside space and nowhere to escape from any new comer does NOT want a 'friend'.
Then you think you can bitch and moan about the prices people want to charge for cats - you've no idea the money they have, or have not, spent in raising their kittens!
And where is it you'll be taking the new kitten (or the old cat) when it turns out they wont live in harmony and instead fight or piss and shit everywhere?
Yeah, that rescue you ignored in the first place, or back on Gumtree.
If you wanted to make this work, you'd be looking for rescues that rehome cats who CANNOT go outside at all who NEED indoor, flat type homes, perhaps due to illness or disability.
You'd be asking how to settle in a new cat, and for details of cat behaviourists to try to ensure this works as smoothly as possible, and you'd have the rescues back up there in case the newbie didn't settle despite your best efforts.