shut off valve behind toilet wont shut off

Hi, I have been searching the internet for some advise on how to fix a problem I am having. I live in apartments and all I need to do is change out the old toilet parts in the tank as they are very old and gross. Also the tank is now a slow fill takes about 30 minutes to an hour between flushes to fill which happened in a snap, no signs of toilet illness. Anyway it is a simple fix that is now halted by a stubborn shut off valve that is stuck in the on position. Normally I would just call the land lord but I want it fixed not further broken or just jimmy rigged....they installed my front door lock upside down so not the brightest repair crew. I have some exp in fixing simple things around the house but I have never come across this problem. The valve is probably about 20 or so years old from the looks of it and the pipes heh even older I would place my money on it. I was informed to put something like a liquid wrench type stuff on the valve stem to help loosen up the seized parts however its been about 4 hours and I have applied it about 4 times then when in after an hour to wiggle the valve it is showing signs of giving but I just want to see if there are any others out there that might can show some light on this so it goes a little faster. I just want to fix the toilet before christmas comes again Sorry for the long issue but I just want to avoid Ideas I am already doing. And yes I know the valve needs to be replaced but I have no clue on where the main water valve for my apartment is or even if I could turn it off if I found it as well.