I've just deployed my first rack mount kit for an MX68. I got it from a company called rackmount.it, and these kits are awesome. I would highly recommend them. They also do kits for the MX64 and MX67 (and MX65 ... but you probably shouldn't be buying new MX65s).
There are no local distributors of this product so I got it off Amazon.
I have no affiliation to this company and I get no financial reward for this referral. I simply think they are great. I really wish Meraki would OEM them and offer them for sale as well so I could buy them more easily.