When you “bake” rock in the sun you are drying it completely out. The problem with this is when the water evaporates out it leaves behind DOM (dead organic matter) which is made up of detritus, dead algae, dead sponges, dead aptasia, dead etc, etc. Then when you add this “baked” rock to your tank you are leaching all of this DOM back into your water column which in turn will leach out PO4, N03, and so on and so forth. An by leaching I don’t mean it will all come out in the first few months and your skimmer will remove it all over night. You will have very large “pools” of PO4 for some time which will affect the color of your corals, and help fuel G.H.A. blooms.
And as far as finding a good place to get rock I know of a few but you have to be willing to pay for it. Just like everything else in this hobby you get what you pay for.