beckhouse2017 It's not possible to purchase worlds through the Sims 3 website anymore, at least not the ones that cost real money rather than SimPoints. But you can buy them through a regular EA sales page, for example:
https://www.ea.com/en/games/the-sims/the-sims-3/buy/addon/the-sims-3-dragon-valley
https://www.ea.com/games/the-sims/the-sims-3/buy/addon/the-sims-3-worlds-bundle
If you don't see the world you're looking for, google "buy Sims 3 [world name]" and it should be the first or second result. Add "gold" to the end of the search if you want the Gold edition instead.
I didn't check every world, but it looks like the bundle and the standard and gold editions of Dragon Valley are 50% off right now, which is normal: the EA App sales are also available on EA websites.
When you make a purchase, you should get a product code (or multiple if you get a bundle). Register the code(s) at http://www.thesims3.com , NOT through the EA App, which seems to auto-consume codes without adding them to the Sims 3 site. This is a problem because the worlds can only be installed through that site, not through the App, so if they don't show as owned, you'll have no way of downloading them.