I don't know of a vendor for that part, but I haven't looked for it, either.

If I understand you correctly, you are missing the lower nylon strap, PLUS the male plastic part that mates to the female Quick-Release buckle which is attached to the upper part of the straps.

I'd ask for either the missing parts. or your money back, as they are unuseable as sold to you.

The plastic part on the lower part of the strap that couples with the upper part of the strap is unique to MOLLE gear, and unlikely to be found being sold by plastic parts vendors, IMHO.  Again, I haven't looked for it, though.

It is possible that mfrs of molle gear, such as Eagle [i]might be willing to talk to you, so that is a possible approach, also.

Good luck.

The plastic parts might be made by ITW Nexus, and Eagle has an assembly plant in New Bedford, MA.  

Do some googling, and a few polite phone calls.  I've done the same, and if you treat the person on the end of the line right, they will help you.

If they do so, offer to write a note to their supervisor on their behalf.  That helps, but offer after goods delivered.