Hi I know that it may be fairly easy to assemble coaxial but I prefer professional made cable, that I know works right when it's for measurements I can rely upon.
I do also know that ebay, aliexpress and other sells a lot from China, but it sounds like a gambling to buy from these shops, have seen RG58 with only a single thin thread (maybe 30 AWG) and also seen cable where it turned out to be some cheap TV coaxial cable, sold as high end cables.
I have looked at DigiKey and Mouser but I could not find RG400 with straight BNC male in both ends.
Where do you buy from to be sure of quality?