Hi Christopher,
I'll give you the short story first and the long story second
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First, try to find the MIL spec document itself (MIL-C-27072) for the cable. I'm not sure if these are available online or not. The "/57" part is called a "slash sheet". It's been 5 years since I worked in the defense business, so the info that follows may be a little vague. The "slash sheet" is going to be either a separate document or an addendum to the main spec. So the MIL-C-27072 document will describe a family of cable types, while the /57 describes a specific member of that family. If you can find the spec and the slash sheet, the slash sheet should cross-reference the vendor and a generic, commercial part number. This cross-reference is the so-called "QPL" or "qualified parts list" for that MIL spec part. Buying it to the MIL spec part number may be impossible or extremely expensive if it is possible. I'll describe the reasons why in the "long story" that follows next. Try to buy it using the generic, commercial part number.
Up until around '94 or so, all defense contracts were written with the requirement that the contractor purchase their electronic parts to MIL specs. IOW, instead of buying an LM741, the contractor would be required to buy a MIL-yadda-yadda, which is really just a 741 that went through lots of extra testing and maybe burn-in or other screening, bought from a vendor on the QPL. The process required to get on the QPL was usually very lengthy and costly, so the vendors recouped their costs by charging big bucks for the parts. Around '94 or so, the DoD rolled out the so-called "COTS initiative". "COTS" is an acronym for "commercial off-the-shelf". What the COTS initiative did was specify that all contracts written from a certain point forward would no longer contain the requirement that the contractor buy their electronic parts to MIL specs. This was a cost-saving measure, but also it allowed using more state-of-the-art parts, as such parts would invariably never have a MIL spec for them due to the cost and complexity of doing so.
Because of the COTS initiative, the only people buying true MIL spec parts nowadays (AFAIK) are those supporting repair or maintenance of equipment designed and built to contracts written before the effective date of the COTS initiative. So it's a dwindling market, and I'm sure the parts are very expensive if they're available at all.
The good news though, is that if you can find the vendor and generic part number from the MIL spec document (MIL-C-27072) and slash sheet, and they still make the cable, you can get it for a reasonable price from one of the cable distributors such as Anixter. The trick is going to be finding the MIL spec document itself and the associated slash sheet.
