I'm not exactly sure when they made this available, but unlike the first (large) Ronin, when the DJI Ronin-M Gimbal was first released, it didn't come with a travel case.
I eventually decided on a carry-on sized hardcase myself, but now here's a custom Ronin-M case direct from DJI. If you're familiar with the Inspire1 travel case, it's a semi-hard exterior made from ABS plastic and a custom cut foam interior.
DJI Ronin-M ABS Plastic Suitcase
Designed for the Ronin-M, the outside shell of this carrying case is made with ABS, a material that's both strong and light. On the inside, the highly resilient EPP material is used to help absorb pressure and vibrations. Besides protecting your Ronin-M Gimbal and all its accessories from collisions, the case is also moderately water-resistant.
Material: ABS (Outside), EPP (Inside)
Size: Approx. 51.5cm long x 38.6cm wide x 23.4cm high
Available now at DJI.com web store (found here).