I would say that these specifications are NOT interchangeable without agreement from the cognizant engineering organization.
Here are just a few differences:
1. The size designation are different
2. Silicon content no less than 62% in the MIL and not less than 67% in AMS.
3. The sample size is approximately 60 drams in the AMS and varies with size in the MIL spec from 100 grams to 25.
I would suggest this MRO / FAA repair facility contact a manufacturer of glass beads such as Potters https://www.pottersindustries.com/micro-glass-beads/
I'm sure they can tell them who their local distributor is.