Here are three repositories where you can find Re:Zero Side Stories.
Phantaminum's Translation Repository
Witch Cult Translations Side Story Repository
Re:Zero Subreddit Side Story Repository
Here are side stories Heretic Translations have done themselves.
Beatrice
Beatrice's Everyone is Troublesome, I Suppose!
Beatrice's Everyone is Troublesome, I Suppose! In Fact!
Elsa
Red Snowfall on the Orcos Domain Part 1
Red Snowfall on the Orcos Domain Part 2
Elsa and Meili: Assassin Sisters Diary KILL 3
Garfiel
Gorgeous Tiger Reloaded
Julius
Julius Juukulius' Promise Keeping Notebook I
Julius Juukulius' Promise Keeping Notebook II
Julius Juukulius' Promise Keeping Notebook III
Petra
Petra's Emilia Camp Struggle / Page 1
Petra's Emilia Camp Struggle / Page 2
Petra's Emilia Camp Struggle / Page 3
Petra's Emilia Camp Struggle / Page 4
Petra's Emilia Camp Struggle / Page 5
Lone Wolf of the Slums Felt-chan's Noisy Days
Puck
Great Spirit Puck’s Nyan Nyan Sunny Day