I think most of the Fiestaware is considered "openstock" so you can buy it one piece at a time. You can find sets, of course, but everyplace I've been that had the sets also had the individual pieces available.
I think eBay or other type of auction site could be a good place to start your search after checking out prices in the online catalogs, of course. Unless you want items that are definitely "vintage", you should be able to get some pieces by auction for less than you'd pay retail.
I don't have any Fiestaware myself but have been tempted to get some in a variety of colors as I really like the "mix and match" style of place settings for casual dinners. Good luck!