How do you get a mystery pack of silly bands?

You can purchase YoVille Mystery Packs in the YoVille Gift and Flower Shop. The prices are 1,000 YoCoins, 2,500 YoCoins, and 5 YoBucks.

Though it is very unlikely you'll get them, the rare prizes are as follows:

1k Pack: Toy Train Set

2.5k Pack: A Unique Toilet

$5 Pack: Vintage Projector that plays a short YoVille Clip