Load up the game and select Toy Box. You can then either choose My INterior or a Toybox.
Once that has loaded click start, choose Toy Store, ToyBox and then scroll to the bottom and select INterior Decorations.
Then simply scroll through the pages and buy 50 items. They vary in price from 100 sparks to 1000+ for the rarer items. There's more than enough cheap items for sale so don't worry about collecting too many sparks.
Please note some of the items are needed for feats such as Beast's Ballroom theme and Hulk furniture. So it may be worth factoring them into your purchases if you're trying to get the 90 feats achievement too (The World at your Feat).
I'd recommend doing this after playing through one of the Playsets so you have plenty of sparks to spare.