In life,it's not uncommon  for nations to buy weapons from each other.  Unlike in galciv, the seller doesn't say"would you like to buy uss/hms $ship1", no? How about   $ship2? (Okay they do sometimes, but not normally  like  that).

Instead ...

Country A & B approve (or don't deny) the sale of arms  and the maker builds to order  with  the  two nations negotiating what is desired/allowed within certain boundaries and such.  Good & trusted allies aloathe transfer of more cutting  edge  stuff, while the useful thorn of their enemy  is instead allowed surplus  & older stuff no longer considered  to  be  cutting  edge.

Being  able  to trade x amount of production from you shipyard of tech older than z turns with either  a civilian only or weapons  allowed option  that the recipient  can choose  what they want to build maybe with a tech known for x turns if not known by both parties  restriction would be neat down the line