I have some from all over (Rosemary & Co, Cheap Joes, etc.) but my favorite is a set from Zecchi.it: Pocket case of wood containing 4 travel Marten Kolinsky brushes n. 2-4-6-8. The case is a beautiful little wooden one that fits in your hand and feels good (like a smooth rock feels good to hold), but it’s not as small or as thin as metal or plastic cases. I also like the way the brushes stand up in the case. Though I often prefer to work with synthetics in larger work, for travel, I love the Zecchi brushes. For something smaller, pocket or mini palette size, the tips in comments above are excellent.