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