A/C leak

I have a 1990 old 98, the AC has been retrofitted from R12, but when i turned the air on a few days ago, i discovered only hot air… so i went an bought a recharge kit, and put it in, it did not get cold… so i poked around and noticed that the High side value stem was a little “off” and could hear a leak… so i went to 3 separate auto parts stores to try to find a replacement. all with no luck, the folks say either they have no way of knowing what stem it takes or was just playing dumb… either way - one store had a stem - but could not tell me if the stem was for suitable both the high and low side - as the package only said for the low… are the stems the same ? or can someone point me in the right direction to ask the correct question/s to get the stem? please help!!!