It's likely your current kitty will become friendly with a newbie without too much trouble - they younger they are, the easier it is.
A few years ago, I brought two "new" kitties home (one was a year old, the other was two) so my 10-yr-old solo kitty could have some friends; her previous lifelong companion had died the previous year.
The two I brought home knew each other but not well (breeder rejects that needed to be adopted out). Once I got them home, the older adoptee (female) made friends with the old lady of the manse with surprising ease, and the two of them quickly grew to hate the younger, male adoptee. Mostly because the male was (and still is) a non-stop lunatic - endless energy, just wants to play, but his roughhousing scared the pee out of the two females and many shrieking, hissing chases ensued.
It took a full YEAR before they could be in the same room together. Now they all
play together - even the old lady joins in the play chases and faux fights.
Moral of the story: even if it seems futile at first, there's still the possibility that cats who do not get along
will learn to like each other.
Good luck!