Hi TinyChild,
You have to look for AU Plugins (Audio Unit - this is the Apple Version of what others call VST or so. Make sure if you buy or download stuff that it is the AU Version).
Have a stop at
http://www.native-instruments.com and look for the
free Kore Player. It will give you an idea of the possibilities. There are also some demo versions. Many prefer the Kontakt sampler over the EXS24, although you can also pimp your EXS24 with new sample packs that are available in many different formats, maybe on sample rom CD's or as downloads.
At last, you'll have to decide what you want and what fits your type of music. If you are making Electronic Music, go for Synths and Loops, if you are making Rock Music, look for Guitar and Vintage Simulations&Amps, if you like classical music, you should have a look at the vienna symphonic orchestra sound library (VSL).
But i strongly recommend you start with modifying and learning the instruments&synths that come with logic. Use them to their limits and you'll really know what you need most to expand your sound capabilities.
Fox