I have two CRS309, and I want to fix them in a 19" rack, but I lost the ears for them and I don't know if I could buy them separately.
There is this page: https://mikrotik.com/products/group/accessories
It mentions some accessories that are available separately (often they are included with products as well).
However, these are not on that list. Probably the easiest is to ask around of someone has bought this device and has not mounted it in a rack, and wants to give them to you.