While the supply items you receive from Supply Capsules and Supply Boxes are primarily based on chance, you can increase your chances of getting rare items by playing higher boosted rooms. The higher the rarity, the more difficult it will be to receive.
If you’re still having difficulty, you can ask other players for help by requesting a trade or gift. To send a supply item to another player. Click on their profile and select “Gift”. You will then be able to select the item you wish to send.