If you want an old time P1 case, your going to have to do some modding. As you know, they were made for AT motherboards, not ATX. You'll have to change the mounting back-plate and cut the back of the case to accommodate the ATX port layout. You'll also have to change out the power button and probably a couple more things. You should be able to install a newer ATX power supply in any full-tower case though.
It would be best to find, as retro looking as you can, a newer ATX full tower case. In my quick search I didn't find much, but there are still some like this
one. I did however find some really cool retro mods, like old radio cases and even one that looked like an old furnace.
Brian.