If you're missing key stickers, there are several ways to acquire them outside of Monopoly Go Stickers events:

1. Purchasing Sticker Packs In-Game

Scopely frequently offers premium sticker packs in the Monopoly GO shop, which can be purchased using real money or event currency.

2. Trading Stickers with Friends

If you have duplicate stickers, consider trading with other players. Many players exchange stickers in online communities or through the in-game trade feature.

3. Buying Stickers from Third-Party Marketplaces

Some online platforms allow players to buy Monopoly GO Stickers directly. However, it's crucial to use trusted sources such as MMOexp to avoid scams or violations of the game's terms of Monopoly Go buy Stickers service.