Is there anywhere were I can buy SET of decent replacement chainrings (48 38 28, or 48 36 26 ). I't appears that to be a lot cheaper to buy a whole triple crank set, I don't want or need?
Yes, Spa cycles stock a good range of Stronglight chainrings which are very good.
But, as you have already pointed out, it's often cheaper to but a complete chainset and end up with a set of crank arms you don't need.