Summit Racing is always a good place to start. I bought my subframes for my 76 there. I bough the bolt in style and then welded them for reinforcement since I don't see any point in removing them any time down the road. I think I paid a little over $100 for mine. It's also a good idea to get new body bushings too if you haven't already replaced them, because you're going to have to loosen the rear body mount from the subframe to install them.
As for the 4-link, I was looking into that for my 76, but my research has lead me to believe they are specifically for coil over suspension. If you're using rear leafs, then you'll probably want to look into a set of Lakewood or Competition Engineering traction bars, and they go for $100 - $150. Or for a little over $300 a slide-a-link kit, also available through Summit, can be installed and pretty much acts like a 4-link for a leaf suspension. Don't quote me on that though, there's other people here that probably know more about the 4-link than me.