Hello @scarlett1913, welcome to TSM help forum!
To answer your question, as far as I know, any items that you see that offer a 75 point reward at a party (like the juice bar, bucket, basketball hoop, grill, etc) will have come from a special event. The only exception I can think of is the hot tub, which was offered through an event, but can also be purchased through the Surprise thing for simcash. The good news is that not only do those happen fairly frequently, but lately the powers that be have been rerunning past events. So if there's an item that you see at parties that you'd really want, chances are good that you'll eventually get the opportunity to acquire them.
You can pretty much count on there being a live event on or around any big holiday, like Christmas (Winterfest), New Years, and Halloween. In the past they've also done events for things like summer and back to school, where you can get cool clothes, hair styles, and sometimes items for the home that may or may not offer 75 points at parties.
Have fun, happy simming, and don't hesitate to ask any other questions you may have.
