go to the "impound" icon on your map and search a bit and you should find some. just don't go past the "authorized personnel only signs" :(

still looking for hidden weapon spots. haven't found any yet. maybe rockstar removed that feature

also still looking for helicopters. i've made it to the roof of quite a few buildings and have even found a few helipads but no copters just yet. i did manage to steal a private jet from the los stantos airport (it was one of the first things i did in this game) but almost immediately crashed it into the ocean because apparently franklin isn't that good at flying