Craft weapon parts are inside the engine rooms, so getting them to drop is a matter of hoping said rooms are still intact after you shoot down the shipping; and if they are intact, then make damn well sure they're not exploded during the ensuing fight. Alternately you can assault the shippings while they're landed so all the engines and thus part drops are intact, but most shippings with the really good craft weapons usually don't land at all.
But if you want guaranteed implosion bomb launcher parts, go after the Marsec viper fighters; you'll always get some because the parts apparently aren't in the engine room, so there's no chance of them getting destroyed along with the engines. You may have to wait a long time however, as viper fighters only show up late in a campaign.
