For a story and keeping two of my households total funds the same as before, I wanted to avoid having a Sim move out just to move back into their old home on joining/marrage. But I couldn't find any mods for it from where I searched. Does anyone know of a location for a mod that does something like that?

I had that happen without mods, just got the Sims joined/married and then don't choose to ask to move in. Works for me. Is that what you're looking for?

Ah, I never remembered it being possible to have an option there on move in status, It is exactly what I was initially looking for! Thanks!

If it doesn't work you could always use Sim Blender or any hack/mod you have to set their relationship as spouses.

I did a couple of playtests and for one couple I simply click 'x' when the move-in option appeared and no moving in happened, they still got married and for the other it just automatically gets them married and moved in together, no option whatsoever. I don't know why this happens. Sorry about that, @Burnziie .

(Edit) Just realized it was a bug in my oldest neighborhood (still I don't know why it happens), never occurs when I start a new one- my bad!

This trick can be done with Sim Blender. (or any other similar tool).
IF sim A wants to throw wedding party on their lot and sim B lives with anyone else: Use Sim Blender Teleport tool and summon sim B to sim A*s lot. Use Sim Blender Family tool and move in sim B. Throw your party.
After party go to sim B lot and teleport and move in sim B back.
If sim B lives alone, you need someone to keep this household. You can use just passerby townie. Go to sim B lot and PAUSE your game, use family tool and move in passerby. Save. Go to Sim A lot and move in sim B and throw your party. After party on sim B lot move sim B back in and In Family tools select placeholder townie and Make me townie. Save. Exit to neighborhood, so townie can disappear and on sim B lot unpause the game.
You can write down exact numbers of household funds and if somefhing gets changed you can use familyfunds cheat to correct it back.