For the longest time there was some folks selling 80% finished receivers for $25. That would have been perfect for a dummy gun, and legal too. Still see some pop up on gunbroker.com from time to time. Otherwise, do a google search for "80% FAL receiver", you should get a bunch of hits.
Parts sets are getting stupid as well, with the new barrel importation ban in effect. I'd tuck one away, low end complete kits also show up on gunbroker for $125-$200. If you're not going to make a working gun, then the bore and overall finish probably wouldn't be a concern.