Targon can't be contracted like other products on the main worlds in that sort of 'set it and forget it' method since the corps that produce it are on a different world. Having said that, I've got a couple options that I can offer.
If you want some direct, obviously a manual trade for a large enough quantity to get you through for a while before needing more again. For 2 battery and 2 missile corps, That's just shy of 1k per turn so ~ 180k per month would be full supply, I'd have no problem providing that in direct trade and you'd just likely have to DM me (discord is best place) when running low and I can top you off again.
Another idea but it relies entirely on my memory and I do have tons of obligations outside of the game, is just make a big manual order with your country at your convenience, and I'll make a conscious effort to try to go through each of the worlds over each weekend and sell down the TA mining products to zero out the current demand on the worlds. That one is probably more reliable, I can easily just set myself a timer on my phone to go off on Saturday that will have me shuttle over the gathered resources and then on Sunday go through each world (I have an enterprise on each) and sell down the demand on each.