There is some decent equipment that you can loot long before you are allowed to buy it but none of that is special like Rowlett's Revenge (except for the Nomad guns, of course).
There is not much lootable post-campaign equipment that you can actually use during the campaign, only the Hornet cruise disruptors and Swatter mines that fall out of Sabres (and Titans probably). The Swatters have only marginally more bang than the Drillers that can be bought before mission 9, and the Hornets are not very useful in the campaign either. Neither has even remotely the appeal of the Rippers that T-Hawk has mentioned already.
Now, if you could jettison a couple dozen Lancer missiles near a capital ship and blow them up with gunfire ... That would be another story entirely!
(The class 7 Lancers are dropped by Red Hessian Sabres and Rheinland Junkers.)
Some decent lootable mid-level kit and some sources:
o class 4 Pyros Turret Type 1 - Balboa wreck in California
o class 4 Pyros Type 1 - Outcast Daggers (best source probably Magellan)
o class 5 Catapult missiles & launcher - Avenger wreck in Texas, Unioner Falcons (Bering), Outcast Sabres (perhaps also Hogosha CSVs, but not sure there)
o class 5 Pyros Type 2 - Balboa wreck in California, Lorenzo wreck in Leeds, Outcast Daggers (missions in Tau-31)
o class 5 Tarantula - Xeno Hawks/Falcons in Hudson and Kepler
o class 6 Adv. Tarantula - Lonestar wreck in Bering
o class 5/6 Vultures (decent damage, incredibly energy-efficient) - Farmers' Alliance ships in Kyushu and elsewhere
The guns of the Corsair Angelito series are also nice and very easy to get (just shoot a couple of Legionaires or Centurions) but they are not as convincing as the Outcast/Xeno kit.
Note: you have to be pretty rich before the enemies start dropping high-level equipment reliably (net worth $200k should be okay for class 5) but during random missions the loot drop chance is higher.