If you live in California, Idaho, Montana, Nevada, North Dakota, Oregon, Utah, Washington, or Wyoming, and shop at Albertson's, you will notice they just started a Monopoly game, with prizes from $5 to $1,000,000.
Don't bother collecting pieces. Just look for these rare ones. The others are easy to obtain (or buy very cheaply online).
Some pieces are semi-rare to go along with a rare one, making the game very hard to win. I got a rare $5,000 piece in 2012 in a different game, but I never found someone to trade/buy the semi-rare one, so it ended up being worthless.
Here are the pieces you're looking for. Where I list two, it means that one is rare and one is semi-rare, so keep both (not sure which is which yet):
$5 Grocery Gift Card - A889
$10 Grocery Gift Card - B887
$15 Monopoly Board Game - C882
$25 Cash - D879
$25 Grocery Gift Card - E874
$50 Grocery Gift Card - F871
$100 Grocery Gift Card - G866
$250 Cash - H864
$250 Grocery Gift Card - I860
$500 XBOX One - J854
$500 Apple iPad Air - K849
$1000 Free Groceries - L848
$2,500 Free Groceries - M843
$5,000 ATV or Camping Package - N839
$10,000 Family Vacation - P832 (rare) and P835 (semi-rare)
$25,000 Backyard Makeover - R828 and R829
$40,000 Motorcycle or Boat - S823 and S825
$40,000 Vehicle of Your Choice - T820 and T821
$100,000 Cash - V812 (semi-rare) and V816 (rare)
$500,000 Dream Home - W807 (rare) and W809 (semi-rare)
$1,000,000 Cash - X803 (semi-rare) and X805 (rare)
If you get one of the pieces where you need two, you are unlikely to get the other. Start scouring the internet for blogs and message boards discussing it, and post like crazy that you have the piece and need that other specific piece.
As a funny aside, in 2012 when I had that $5,000 rare piece, I was contacted by someone who claimed they had the other piece, so I got all excited and asked for their phone number.
It turned out it was some girl in her early 20s from Newhall, CA (about 30 miles north of LA) and she was really confused. She had the same rare piece I did, and we both needed the semi-rare one. While I was trying to explain it to her, she started to flirt with me and attempted to stay on the phone as long as possible. I've been with Ben's mom since 2009, so I politely ignored it and got off the phone. To be fair, I never told her how old I was, so maybe she thought I was somewhat near her age. I also heard a young kid in the background, so maybe she was looking for a new baby daddy.