What is the proper way to bury broken or worn out sacramentals?
Well, the ordinary means of purifying sacred vessels used in the Eucharist is to drain them in a special basin called a sacrarium, which usually empties into a permeated pipe dug into the earth, usually to a depth of about 20 feet.
If a sacrarium is not available, such materials (which also include spoiled Eucharistic materials - moldy Hosts and soured Sacred Wine) are to be buried in the earth. The manner of this burial is not clearly specified.
The manner of disposal of Sacramentals is not defined with more detail than spoiled elements of the Eucharist, but I would imagine that, if simple (but reverent) burial was sufficient for the Eucharist, it would be sufficient for lesser objects.
So, dig a small hole in some appropriate location, and bury the objects. I would think that you could ask a priest if you could utilize the Parish flower garden, and he would not object.