decorations are found in the Shopping Bag icon, under the Tree icon section. if you finish all the main chapter 1 missions you should have 3 decorations and over $10,000. with this money you can easily afford the maximum of 40 of the cheapest object, $75 cedar edges ($3000 total). then you will need a few more objects to reach 50 total. square tulip pots and small lampposts are cheap, also the other shaped cedar edges are pretty cheap.
you should also be able to afford:
Warning: keep in mind that completing missions usually require spending money, which is hard to come by at first, so you might want to save doing this until you have some extra cash. the beginning of the chapter 2 missions required about $8000 to buy a gorilla habitat and then a few more thousand to upgrade it, so i had to wait for some profits and click on my decorations for money a bit to be able to afford to continue.